My turtle is starting to look thick?!?

It's hard to tell from the pictures, but I think he looks pretty good. I generally I don't worry about hatchlings being overweight. They're growing pretty fast at this point, and need lots of good stuff to make strong bones and shells. What are you feeding him? Does he have a UVB light? As long as you're feeding him as much of the right stuff as he'll eat in about 15 minutes, I wouldn't worry. Make sure he has a good calcium supplement. I dip the food in a little bit of water, then into the calcium and feed them from a bowl or off of tongs. I would be more concerned about your other baby. Are they kept in the same tank? There can be some very subtle bullying going on when reptiles are housed together. Sometimes the dominant animal blocks access to the best food, resources, and basking spots. I would try feeding them separately, just to make sure there's nothing like that going on, and that this guy is not keeping your little one from getting enough food. Good luck!