some old photos
CORRECT. Yokozuna portraits from before 1980, a lot of them are thinner (still heavily built compared to the average person). Even back in the 1970s there were numerous thinner high ranked wrestlers (Yokozuna Wajima and Ozeki Takanohana 1 come to mind).
Then steroids happened. Later this year serious steroid testing is going to begin. Basically everybody in the top ranks is on the silly sauce.
Today's small man: The Amazing Ama, 6'2" 270lbs
May 2008 vs. 6'6" 360lbs Wakanoho:
Back in 2005 when Ama was only 235lbs, vs. 400lbs+ Iwakiyama
the outdoor matches show that the wrestlers in the pics are probably amateurs and just didn't have access to a lot of food.
Compare this to Ukiyo-e illustrations usually show wrestlers as fat: