GainingLear:
There are plenty of examples of normal people in the 500-600 range who are still mobile just among models. Your demon could probably be looking at 700-800 easily if you didn't want to make it extreme like a full half ton.
Not just still mobile, but very agile too.
For example, look up Boberry, she is almost 600 pounds, but can give a thin girl a piggyback ride and run with her on her back, and she can kick higher than her head, something even many thin people cannot do.
