That photo above must have been before shooting started. I recall a panicky memo from Roddenberry to Justman about how much weight Shatner had put on between seasons, and, from memory, Shatner agreed and went on a crash diet. And it worked, he slimmed down a lot.
Kirk's appearance was very different in Season 3 - he appears a lot older in the face (no doubt due to the personal dramas Shatner was having that Melakon mentioned). His hair/wig is darker and thicker, and even though he looks pudgy in the face, around the belly (in the season proper) he looks in the best shape he's been in since Season 1.