That was one of the ongoing inside jokes on Voyager. The EMH & Janeway have said in passing about the bridge: "It's much bigger than I thought".
Why in the world is the Captains Ready Room on Voyager so damn big? What did she need all that room for? That seems like a waste of resources IMO. I just don't see the point for it.
On the technical side, I think it has to do with the type of ship Voyager is. It's a scout ship, notice how it has no labs, no areas for botany or studies of any kind. All Voyager has, is a bridge, sickbay, engineering and cargo bays. Most of the ships primary functions would take place on the bridge. Seeing as how is was a scout ship designed for deep space, Janeway and her crew would be out of range of other ships for long periods of time. Her ready room was designed to be her private oasis. A place that was multi-functional for not only work but relaxation as well.
I think the ship will have Science and Engineering labs, as well as labs for Botany etc. We have seen at least two labs seen on the ship in different episodes and the ship did have a Hydroponics bay. If they didn't have labs where would you put the Science and Operations crewman on duty when there not in either Engineering or on the Bridge Science station (of which there is opnly one)?