The number-one factor that determines flying the big planes is seniority.

Many more senior pilots (but not all) like flying the big planes, as hughw wrote, because they can do fewer (longer) trips a month. Some more senior pilots, though, prefer a larger number of shorter trips, often so they can be closer to home.

Finally, a lot of it depends on the way the fleet changes over time. Once, I sat next to a CO pilot deadheading and he told me that, although he was currently a 737 captain, he used to fly the 747's (as a FO).