My Shannon has recently taken to lying on her back like that, exposing a large amount of white undercarriage. She’s about double the diameter of your cat!
If your cat is fat, then put him on wet food. My big old tom cat is diabetic, and gets two shots a day. I put him on mostly wet food, and he has lost weight over the last few years, and is scampering around like a kitten. I have also been able to reduce his insulin.