I think even though Sumo's are big they still have a balanced diet; i guess it's something to look up.
EDIT: A La Wiki!
Rikishi are not normally allowed to eat breakfast and are expected to have a form of siesta after a large lunch. The most common type of lunch served is the traditional "sumo meal" of chankonabe which consists of a simmering stew cooked at table which contains various fish, meat, and vegetables. It is usually eaten with rice and washed down with beer. This regimen of no breakfast and a large lunch followed by a sleep helps rikishi put on weight so as to compete more effectively.
Rikishi is a rank of Sumo, so guess the time of day or how many meals they get depends on the rank, but the meals are probably the same. The downside to Sumo is that they live until somewhere between 60 and 65. They're prone to diabities, high blood pressure and heart attacks.