Where did you see any evidence that there are bigger ships, to give you this impression ?
USS Vengeance, for one. But also the existence of TOS fanon where the Enterprise wasn't the largest ship in THAT universe either; the three-nacelle Dreadnoughts (which are partially canon via TSFS bridge displays) were significantly larger, as was the Proxima class
from "Star Trek Legacy".
Prior to that, there was the fact that the only other ships that accompanied Enterprise to Vulcan were all much older-looking Kelvin kitbashes and all similar size. It stood to reason that Enterprise was the odd man out in that group and therefore newer generations of starships were being built out of similar "modules" as the Enterprise, also being of roughly similar size if not slightly larger. So I figured the Abramsverse Proxmia class would be scaled accordingly, even if its actual design might be dramatically different.
Which, again, turned out to be a correct assumption vis a vis USS Vengeance.