According to Zahabi he was 183-184 before this camp. So no he is not a big WW.
Having stood right beside him I can tell you that they exaggerate his height as well.
http://www.bloodyelbow.com/stories/5
We can also look at it this way: he has one fight at 185 (not a lot of superstar talent) against a MUCH bigger (taller and heavier) opponent in Anderson Silva - who walks around at ~220, or a bunch of fights at 155, a much deeper division. And yup, having met him, I can confirm GSP is not a big dude.
Further, if he moves up to MW, there's a chance he can never move down again. I look at the boxers who somewhat recently added muscle to move up and were either unable to move back down (Bernard Hopkins) or if they did, were never the same again afterward (Roy Jones, Jr., Antonio Tarver.)