I've been on the
Phase II sets, and they are a remarkable achievement in every way - craft, research, attention to detail and practicality to the extent that the space they work in allows. If they get tired of making movies, those guys can open a museum.
That said, the
apparent size of a set does have a lot to do with both camera angles and lenses (and lighting, lighting, lighting). The
Phase II bridge is (or was) "buttoned up" rather than having large sections that can be pulled out of the way (they may have ways of removing viewscreens or something to get behind the structure for longer angles; I can't speak to that). So it's
possible that there are wide angles you're used to seeing in TOS that make the bridge set look bigger sometimes than are used on Phase II - but I don't know, the bridge looks pretty much the same to me as it does on TOS.
I really suspect that the
apparently smaller size of a set like Sickbay on screen has to do with how closely together some of the sets are necessarily constructed and resulting limitations on camera angles. Per Doug Drexler, the original Sickbay was built on an open plan, "three camera sitcom" layout that enabled the crew to effectively shoot it from outside the set itself and to dolly the camera quite a bit.
A careful viewer can easily be mistaken in estimating the size of that set as being smaller than the original - it doesn't matter on screen what something
is, just what it looks like. Sort of basic to movie making, that.