I’d agree with TJ, if you can be arsed, raise a dispute, state your case and let ebay decide. If you described it as a medium and it says Medium on the label, then it is as described. Even if you didn’t state a size and he didn’t ask you, then I’d argue that he can’t claim not as described, because you didn’t state/describe a size. The fact that he can’t get his beer gut into it is his problem TBH and he should be grateful that you are prepared to accept a return (but at his expense IMO).
I bought a jacket that was described as unisex medium only to find it had a size 12 label in it, which maybe is a women’s medium but def not unisex medium. Something like that might be grounds for a dispute, or if you stated a size contrary to that stated by the manufacturer on a label or you mislead him in a communication.