To be fair, Rock went through a period of slimming down from around 2006 - 2009 and that's when that prior photo was taken. Dude is huge now though.
He was bigger in his early WWF career as well, he noticably slimmed down from his NOD/Corporation days to what he was like during 2000/2001.
Not saying that he is/isn't on steroids but he has fluctuated. For what it's worth I think the slimmer version looks much better and for christs sake grow some hair again, The Rock looks like a Polynesian Steve Austin these days.