But yeah, it bothered me that Janeway has a huge spread over Picard's rather cramped space.
Of course it could simply be "design" differences. Voyager at the beginning was a brand new ship. I have no idea why anyone would care in the first place. The design of sets has more to do with visually pleasing shots than worrying about who is more deserving of a bigger ready room. After all Kirk didn't have one at all.
If you look at the plans there really isn't a lot of differences in the size of either one.