Why not build bigger dreadnoughts?

I accept that you're just really fishing to substantiate your idea that building the Nexus should have been "too expensive"; so you're missing the point about the Treaty of Farixen being a MIRROR of the Washington Naval Treaty of WWI... one of the consequences of which was the loss of interest by nations in building dreadnoughts, which were limited under the treaty, and prefering to build carriers, which were not limited.  Being more interested in having a "larger navy" and just a few "bigger ships" resulted in them pouring their research monies into way to make the carriers better... which, combined with their disadvantages as opposed to carriers, ultimately, led to the dreadnoughts becoming obsolete.  That ME's dreadnoughts have some disadvantages (maneuverability, enemies being able to track fire from a long distance, unable to land on planets to discharge, etc.) and the Treaty of Farixen means that the races, other than the Turians, can build only a limited number of them.  This means they probably switched their research away from the "bigger gun" scenario you're trying to push here.

We don't know how large or how many carriers the Alliance has or the Asari have... we aren't shown them (because they tend to stay back of the battle and as they deploy their ships... so they are not shown in the battle over Earth).  For all we know, they may have many of them and they may be quite large.  (The Wiki does say they are dreadnought-sized).  Dreadnoughts, like the Destiny Ascension seem to be in short supply - Again, because the dreadnoughts (i.e. the ships with the large guns) are limited by Farixen... but, as I said, I don't even think the DA was the Asari's only dreadnought since we are shown a second similar looking ship in the battle over Earth.  It seems unlikely that the Destiny Ascension deploys fighters... we're never shown it doing so... even before the 5th Fleet moves in to save it (if that's what the player decides) in ME1.  The geth dreadnought is the only dreadnought we're shown that deploys fighters.

That still doesn't change the fact that they are building navies with the need to cover a large galaxy with a larger number of ships rather than trying to cover it with fewer larger ships.  The Nexus, on the other hand, is the only one they need to build.  That they build 4 much smaller Arks shows that they don't want to put everyone whose going on one ship (just in case), but the plan was obviously that everyone would ultimately stay on the Nexus until viable worlds were found to colonize.