I think my dog is some combination of lab/pit bull/boxer. She is about a year and a half old and weighs over 80 lbs, and has an underbite. Her neck has gotten several inches thicker in the six months I've had her and she's gained 20 lbs... I'm beginning to wonder if this dog will ever stop growing or if I should buy her a new collar... perhaps there is a mix I'm not considering? I will post a picture as soon as I figure out how.

Here are the promised pictures, hope I got a good enough angle of the face and body shape. The face pics are from when I first got her, the body pic is more recent.

I don't see pit...

Boxer maybe...
Lab... Doubtful..

Maybe part mastiff..

I see lab for the short black hair and half the face, and maybe newfie based on size...
But there's no point in this exercise. Most mutts have so many breeds in them that even DNA testing is not going to tell.

I have friends who had an enormous golden retriever, purebred, from a family tree of large goldens but none that large. People walking by would insist he wasn't a golden (it got really annoying after years), yet he really was. He had a huge head and jaw and was probably at least as big as your dog. They CAN vary!

Any type of black dog is automatically lab mix

From the picture, the hair is too short and thin.

Any type of black dog is automatically lab mix

From the picture, the hair is too short and thin.

She definitely doesn't have a lab coat, it is not double layered, for one thing, and you can't see in the pics but she has a white patch on her chest, but her behavior is very much like a lab and her longish nose and ears look like a lab. I've seen pictures of English labs that look just like her except the coat and coloring...

Countless black lab mixes have a white patch!