Things can be a little cramped when stuffed inside a spacecraft. Not only are your crewmates taking up room, but toss in supplies and lots of gear for good measure!
And if you're headed for Mars, elbow room may be at a premium for such a long voyage. The good news is that living and working in microgravity does create opportunities for astronauts to expand their environment because they are not constrained by being bound to a "floor."
But NASA has yet to map how astronauts take advantage of weightlessness to expand the useable area of their vehicles.