I don't know why Costa was mentioned, he's a MW so that doesn't matter. People make the mistake of taking GSP's walk around weight & say therefore he's small. GSP is in great shape yr round, GSP, even at 194 lbs in the cage vs Shields, was still in great shape, he wasn't fat or flabby at all. 170 was the best division for GSP, he'd be killing himself to make 155.
It doesn't matter if 2 guys have a similar walk around weight, what matters is what weight are they at in fighting shape. A lot of fighters get fat or out of shape when they don't have a fight signed. GSP is super disciplined, he stayed in great shape yr round. When GSP was in his prime, no one called him a small WW. He has very thick legs & wide shoulders too