You could've fooled me. You basically said "You're fat" and then turned around and said "I'm just kidding." It wasn't that funny and it was pretty rude.
It was uncalled for. That's like me going up to a kid in school, punching him and then 5 minutes later say "It was just a joke." Yeah, words aren't as harsh as a punch, but they can hurt pretty bad (trust me, I know), especially if you make fun of a guy for his appearance (btw, I think Turtlekid looks fine).
There are other ways to be funny, man. Making fun of someone's appearance and then claiming it was all in good fun isn't really one of them; although, I suppose if you were really good friends with Turtlekid, he might have thought it was funny.