Naval forces in Mass Effect

I'm not sure what the difference between a Corvette and a Patrol Boat would be in a space bound setting.
The ME verse the doesn't seem to be distinguish between Corvettes and Frigates / doesn't use Corvettes.
(Probably because the main difference historically was the size, and the seaworthiness. A military space ship that is unfit for prologued space flight seems not very useful.)

About the MEverse relying on large ships; I'd like to point you to the ME Codex, specifically Military Ship Classifications, and the entirety of the Space Combat section.

As for the reason we don't see much of that: Rule of Cool.
Also Reapers are pretty invulnerable to anything without at least cruiser length main guns.

I'd imagine that things like EW craft and bombers have just been phased out. Why have dedicated EW ships when you can stick sensors capable of doing the job on every ship. And as for bombers, the games don't portray it very well but an orbital strike from even a small ship would be devastating.

I'm sure there are things like refueling ships and whatnot but we don't see them because there's never any need for them.

Well, I guess one issue is the "big guns on small boats" premise. That works for something like missiles and such, but in the MEverse, basically everything other than ME field augmented rail guns are obsolete. So the rule there is "the longer the barrel, the more punch it has". So a bigger gun by definition requires a bigger boat. Hence the ship classification as laid out in the codex.

Of course, there are exceptions like Javelins and disrupter torpedoes but here, small one-man craft seem to be the best choice because mid sized ships would be more expensive and at the close range they'd need to get at to fire them effectively, they would be just as vulnerable to GUARDIAN lasers than fighters, so they went with swarm tactics instead of mid sized ships.

Yea, I mean, in the end, it's all rule of cool but this is how I justify it for myself.

One of the planets in ME3 references submarines in use against the Reapers. Can't remember which.