This is Baxter's human. Thanks for the response. I agree that avoiding reference to Baxter being part Pit Bull is likely. Baxter just doesn't have any traits of a Pug, so when I saw pictures of Pit Bull/Border Collie mixed dogs that look so much like Baxter, it just made more sense. Baxter is pretty darned solid, so I don't think his bulky frame is from being sedentary. Baxter says thank you as well. It's important to him to know better where he may have come from. Here's a photo of Baxter chilling;Welcome to the forum.
If I had to venture a guess, I'd think your person has a better idea of your breed than the shelter (which has a strong incentive not to call a dog a pit bull mix unless it's unavoidable, because they are more difficult to place). A bit more activity with your people may also help to show if the "full bodied" is innate, or just from a sedentary lifestyle.