The Enterprise-D, as well as Voyager, had quarters that varied in size throughout their respective runs. Which makes sense... rank, position, importance of visiting person, etc. factor in to who gets assigned what size.
In TNG's "Lower Decks", it's established that Lt. j.g. and higher ranks get their own quarters, while Ensigns (and possibly crewman) bunk at least 2 per quarters. Picard's quarters are obviously bigger than the rest because he's the captain. We also see guest quarters range in size, too. Scotty in "Relics" clearly thought his quarters were massive. Family quarters also seem to be a bit bigger than standard quarters because a family lives there.
On Voyager, it's clear senior officers had their own quarters, which explains why Kim got his own despite being an Ensign. Janeway's seemed larger than anyone else's. (With Chakotay and Tuvok's quarters looking to be very close behind hers, but that still makes sense given their rank and positions.)
I do agree that her Ready Room is very large in comparison to the Enterprise. I don't have a reasonable explanation for it, but someone in this thread mentioned the bridge module of the Galaxy class ship being... well, modular. That's as good a reason as any... maybe the best possible reason. (Real life, out of universe reason was probably that they didn't expect to film quite as many scenes in the Ready Room as they ended up doing... so when it came time to create VOYAGER, the producers decided to make filming easier in there by making it much bigger than TNG's counterpart.)
The one question I always wondered... the XO and Security Chief got their own office on Voyager, but we never saw Riker and Worf get the same. Surely a ship as large as the Enterprise, those two officers would have their own office? (Though I'm pretty sure that Geordi had his own, at least from dialogue in "Galaxy's Child", despite never actually seeing it. And the Chief Engineer should have their own office... it's arguably the most important role on the ship. Which now makes me wonder why B'Elanna never got one?)