What's the significance of orbit diameter in a random orbit sander?
Orbit diameter is related to how aggressively the sander removes material; the larger the orbit, the quicker it removes material (all else held equal).  This article on American Woodworker's site seems to agree.  It also mentions that larger orbits leave more visible swirl marks.

However, orbit size is not the only determining factor in how quickly material removal happens.  Your best bet is probably to find a good review that compares sanders rather than relying solely on the orbital diameter.  For example in this Popular Woodworking article, the sander with the largest orbit wasn't the most aggressive.