We have more Legos than I like to think about around here.
That said, my boys mix everything, so we have 3 large under-bed storage bins for most all pieces, plus some of those "boot" sized bins for specialty pieces (people, weapons (think Star Wars and the like) and wheels/axils, I think). The boot sized bins are supposed to have a lid on them and put on a shelf in their room. They too like to keep every creation they made on display, but I usually give them a shelf on top of their cubbies and/or a shelf in the closet (about 2' x 2'); they keep the most important ones there and the rest are put back into the regular bins to become something new again. Sometimes the Legos will be put in the garage on "time out"
if the boys are having too hard of a time keeping them contained, but if we stay on top of them with the bins (on top of the Legos), and work together to clean up at the end of the day, the big bins under the bed keep most of it contained. Convincing them to pare down their creations is another story, but having a physical space that limits the amount helps.