GSP is a big WW he is really thick muscularly for his frame.
exactly - he's a pretty short guy for WW. Everyone weights in at 170, and they are all cutting from 190ish and then rehydrating to about that same weight.
If in shape, the shorter guys are going to be thicker. Hence Thiago Alves's nickname "pitbull". He's probably the shortest WW in the UFC, and probably one of the thickest when in shape.