This has already been discussed at great length on the forums. There are many threads about it. It has to do with how the 3D engine's camera works, and the need for there to be plenty of room for missions involving large numbers of players. (The DS9 Fleet Action comes to mind.)
I don't buy it either. Everything seems better scaled in Champions Online and that's the same engine.
Even Champions Online interiors are similar scaled. It's a lesson that Cryptic leaned from their first maps they designed for City of Heroes. There are cave interiors which are very narrow. CoH's team size is 8 and the maximum spawn size is 17, and there are spawn point location which can put 2 or 3 spawns within possible aggro distance of the team. Mass combat on those maps was just frustrating as hell. If you won't trusts the devs word on it. Trust my word as a player. I don't like the scaling aesthetics either, but the alternative of bad game play is much worse. Trust me.