Our first mutt girlie Rosie had a huge mass come up quickly in the same area you describe on Cullen. It was removed. Turned out to be a lipoma. She'd had smaller lipomas over the years although much smaller. Vet said while this huge one was benign, it wasn't just round but growing a tail, becoming invasive, and pushing hard on her internal organs. Once removed it did return years later but never got a large as the first time.
Hope your Cullen gets some good news soon.