Short main quest in a world too big?

I don't have Trespasser and I'm playing on hard
I disagree about the Witcher 3, I thought the pacing was just on point. 

Sort of, yes. I don't think the game is necessarily too long, but rather that so many side quests feel like filler rather than feeling essential to complete. You can skip entire areas and still have enough resources, experience, and prerequisites to go ahead and finish the main story. I would gladly have given up a number of fetch quests in exchange for more companion quests, more romance-related quests, more cutscenes, etc. Why would the Inquisitor be the right person to find some farmer's stupid druffalo (I both love and hate that damn thing) for example?

I've played through only twice up to this point, but by trying to be a completionist (and still never finding all those mosaics despite that I thought I left no stone unturned ) I have been way over-leveled for the final fight both times. I haven't had the backbone to try nightmare difficulty, but Corypheus ended up being almost a pushover on anything lower than that by the time I had to fight him.

I haven't gotten very far in W3 yet because so far I'd rather play DA (again), but I thought, surely, Geralt being a way over-qualified exterminator had to be as ridiculous as it got. Finding Ciri had better be worth it if I ever get there, dammit. 

Fluff is the problem. For me it's not really about (over)levelling or
the size of the world, it's just that I don't care about these quests.

And I actually prefer the overwhelming feeling W3 sometimes gave me (and yeah, Novigrad section was, how can I put it, overwritten?) to the blandness I felt when exploring some of the zones in DAI. 

There was a problem if you do all the areas before the final fight with Corypheus. Forget about trials and such, this is about the pacing of the original game where they did not exist. The only time I found Corypheus a challenge was on my first run. I think I had only killed one dragon (in Crestwood) and I'd missed out the whole of the Hissing Wastes, a fair bit of the Emerald Graves, the bit across the river in the Exalted Plains, the last bit of the Forbidden Oasis (since I hadn't found all the shards) plus some other optional quests, so was on around level 19 when I got to him. Then I discovered that all the fun conversations back at base stop once you had finished the main game and I no longer had Solas around, so subsequent runs I tried to do everything before the final battle (except may be dragon hunting) but even without dragons I found fighting him ridiculously easy.

I keep wondering if there was originally meant to be something significant in the Hissing Wastes and the Forbidden Oasis that was then omitted through lack of time.  This seems borne out by the fact that they are both recorded in the Keep.  They seemed to have no bearing whatsoever on the main plot, even though the Venatori were said to be searching for something important. To have stomped round the Hissing Wastes only to find nothing relevant to Corypheus was extremely disappointing. I also found it frustrating that using the mount meant you might potentially miss out on companion banter because it was one of the few places where you might be running around long enough for it to trigger.

So I agree with OP, it did seem the game was overly huge compared with the actual parts related to the main plot or necessary quests connected with the relevant lieutenants. A little bit more exploration to discover things that actually influenced how easy or difficult the final battle might be, would have been welcome. The shards at least netted you some attribute points, although I resented being forced to look through the skulls after I discovered how they were made. We never did discover why the Venatori were so obsessed with looking for elven artefacts, except the obvious one that they are ancient and powerful, and we never did find anything that warranted that amount of interest.

Instead of the split soul dragon solution being simply to kill the dragon, it would have been good if you needed a certain weapon to kill it/Corypheus permanently and the various bits for this were scattered around Thedas. That would have explained the Venatori interest, have given you a valid reason to explore everywhere and entailed constructing it before the final battle, or Corypheus would have regenerated again. That would have been a neat twist.

Personally I think Emprise and Hissing Wastes should have had post main quest content in them. Storylines that involved mopping up the Red Templars and Venatori after Cory's defeat. As it is, those zones feel quite unnecessary. I didn't even visit Hissing Wastes in my first playthru,

There's also the reward of a herbalist or merchant when you free areas of bandits in TW3.
I don't like to use the term fetch quest when the quest has a context and some of them even lead to other content, but yes, in TW3 there are side-quests that you need to get something to an NPC, and still those end up revealing something else. There's also that we have zoomed in conversation making what you're doing more immersive. One of my biggest gripes with DAI was the zoomed out conversations. Not only is the side-quests inconsequential and boring, you can't even look at the NPCs faces up close. Why should I care for their struggle?

No, but I understand why people feel that way, particularly in comparison to Witcher 3 or Fallout 3 or even DAO.

If you sum up all the possible experiences in DAI and then piechart the different types of content, including the "meaty content", which we'll arbitrarily define as any content that has cutscenes essential to it, the segment of the pie with meaty content is less than half of the whole. Something like 20 to 40 hours is a reasonable estimate of the "meaty" stuff in the main game, while the totality of the experience is easily 3 times that.

But here's how I explain, not apologize, for that discrepancy. A lot of the stuff people feel is useless filler is stuff that was added for replay value. You're not supposed to try to play it all in one-go. This is clear both from the structure of the main quest (CotJ vs IHW), the structure of romances and hard-locks, and in the structure of the optional map zones. If you do try to play it all in one go, you're going to get the piechart effect and the "meaty content" fraction is going to seem small compared to your total experience.

If, instead, you work in a little of the optional content -- or none, if you so choose -- you get a really good 20 to 40 hour experience with almost no filler. And the next time you play, if you pick different optional content that you didn't try before, you get a new experience!

Now, in the final analysis, was all of that intentional replay-value design a net mistake on Bioware's part? Maybe. Clearly, there aren't enough clues or guidance to get new players out of the Hinterlands sooner. Clearly, there's not enough of a clue that fetch quests are optional, skip them if you don't like them. There does seem to be an expectation common in the market that meaty side-quests are mandatory and anything added purely for replay value is going to result in lower Metacritic scores. Deus Ex: Mankind Divided is getting some grief along those lines.

I would have cut 2-3 of the zones tied all zones to the plot except for like one bonus zone...though on the other hand maybe not have any bonus zones per se because the Temple of Dirthamen and Cradle of Sulevin might work for that specific itch to.

I think the main story could've been longer, but it would've been fine if the side quests had been more interesting.

On the topic of TW3, I found the main story dragged on just a bit too long. Especially searching for Ciri when they give you several zones to look for her. Obviously, you know already you won't find her until all the zones are complete so that kills all suspense. But each area's story was quite good so it gets somewhat of a pass for that. And the side quests were great so it helped break up the main story. It definitely flowed better than DAI.

This is the part I don't get. Optional stuff is optional. If you don't like maps that are not connected to the main quest, or fetching shards, or anything that's optional, why do it? I have never collected all the Mosaic pieces and have no interest in doing so. But I don't then force myself to do it and then complain about how bored I am.

Is it fair to say that most of the pain experienced with boring MMO-esque or padded out filler in DAI is self-inflicted? Perhaps Joyous, you can help me understand better why you feel you have to do stuff you don't like doing. Is it the Power mechanic?

The reason you try to do everything initially is because you think it must be important. Then eventually the penny drops and you realise it really isn't. This is why I totally ignore the shards now and those yellow glowing circles that I know are just more bottles of Thedas. I'm totally okay with the fact that you can ignore areas if you want, but I wish they hadn't made it seem as though they were important. The Venatori set up all those posts to find shards, so they must be significant to the story, but they aren't. Why are they so bothered about dwarven ruins in the Hissing Wastes? It must be important but it isn't. Then they record in the Keep whether or not you did complete them. This is why, after the first run when I hadn't completed them (or even gone to the Hissing Wastes) I felt obliged to do so because I was convinced I must have missed something really important. It turned out I really hadn't.