Comparing the Ultimate Warrior to a training video I seen of his from the early 90's, he does look much smaller than he did in his prime. However, I do not believe for a second he did not take steroids after 1991. In his last run back in 1998, he looked as big as he was in his prime as the WWF champion.
I also seen Warrior speaking as if he was very pro-steroids when interviewed on the news last year giving the impression as if he's open to re-use if necessary, especially seeing he is making his huge in-ring return in Spain.
Of course he says that, and because most people are clueless, they'll believe him. Still, he has had to bust his ass to be in shape like that regardless of what he takes.