He isn't fat, he's just not naturally athletic and that gets to people. I think you'll see his conditioning improve with games, he basically jogged around most of pre-season. He's never going to be one of those players who hit the ground running because two months off without proper games will not be kind to him.
He's a stocky, barrel shape. Tevez is the same but he also has stamina and running, so no one cares. Higuain is now mostly a final third player.
According to info I can find:
~75kg
~79kg
~82kg
~87kg
He's probably about the same now.
Comparison with a muscular teammate - Pjaca who is 2cm taller is 85kg
Comparison with a fat striker - R9 was 94-96kg when he played for Real Madrid