No, it is not about that. When concerts were given two centuries ago, they were presented in salons, social gatherings and smaller venues than we have today. With the increased space for the audience, we still play those older works, sometimes with a larger ensemble than the one that performed back then.
Are concert halls simply too big now? To me, it simply does not make sense to have mammoth forces just to fill up the stage. Today you can see orchestral rosters that contain as many as 106 musicians, but rarely do they all play at the same time. As goes the venue, should the orchestral complement take into account what the hall can accommodate?
This is a tricky issue. I would appreciate hearing from you with questions or comments on this particular subject.