To be honest I thought it was because of garrosh's exposures to the evil sha gods during pandaria's mists that made his mind completely crazy and gave him the body that resembles a troll. Prior to that, even in the raid on the icecrown, one WOW TBC Gold can see that garrosh was much smaller in stature and thrall was larger than him. He also joined the iron hoard and grew stronger with the iron horde.
He was a hard worker and became stronger because of his "real iron bros from the gangsta scene".
In addition, I do believe that cheap WOW Classic TBC Gold Thrall was reserving his words throughout the fight because internally he felt that there was a problem with him. He believed the same until he had the conversation in Korthia with his mother.