Well, in the context of MITM, it was right around the end of season 5 where bullying stopped being so important to Reese. Certainly it was still the only thing he was good at, but he became involved with "projects" other than simply going to school and beating people up every day. Perhaps he was too afraid to work out anymore after his bench-pressing incident in Forwards Backwards.
In a context not consistent with the show apparently Justin is just small. ;D