Maybe just because they're both turtles who can stand up straight?

I wouldn't call the Ninja Turtles "fat" (even though they eat a lot of pizza), nor the Tuff Turtle. They're both... muscle-bound, the one more than the other. They both have empty eyes, with no pupil and iris. However, the Ninja Turtles have masks (their eyes have a pupil when they put the masks off and they're white btw), the Tuff Turtles' eyes are (somewhat) natural, they're evil, pumped by steroids :P

- JelZe GoldRabbit =:3