Mandalay bay does not have 100% occupancy 365 days per year. They probably don't even have 50% occupancy 365 days per year. But yes, it's about the size of a VERY large hotel.
They also have tweaked pricing to keep rooms filled. The same room at MB is going for $70 on 3/13 (Sunday) vs. $270 on 3/16 (Wed). (To be honest, sophisticated enough that I don't even really understand that. Usually it is the weekends that have the prices jacked.)