I've always been somewhat confused by them. I'm not good at explaining things, so I hope this makes at least a modicum of sense!
In the very first TNG episode, we saw that once a holodeck program is started, one's freedom of movement inside the program is limited to the actual walls of the holodeck room (as we saw when the holodeck wall was hit by a rock). In other words, if you started a holodeck program of, say, Disney World, you wouldn't be able to walk through the entire park, you'd just be limited to the actual walls of the holodeck, right?
Well, why is it we saw some TNG episodes of Dixon Hill (and also in "A Fistful of Datas") where the virtual environment was bigger than the room, and the crew was able to walk beyond the walls of the holodeck?
Also, in the Voyager episode "The Killing Game" they were fighting a World War II battle in the holodeck, and the program was messing up, somehow the holodeck became like two or three stories tall when the room was NOT that big!