There may very well be a few individuals who take steroids, but of the really good wrestlers it is technique and balance that take them to the highest level, and strength of course. The exceptions were americans Akebono and Musashimaru, who mainly relied on bulk, but were able to use that extremely effectively having almost all their wins by oshidashi and yorikiri.
By the way, this is the next "big" thing in sumo, a 22 year old 380lbs former judo champion. He will be the first european yokozuna. Remember I called it;)
http://sumo.goo.ne.jp/eng/ozumo_meikan/rikishi_joho/rikishi_2731.html