LOL! I think if I were an alien, I'd want someone lean and toned. Less fat to be trimmed.
I do think it is a bit of fat bashing in an inappropriate stab at humor. Yet as others have said, I'd just ignore it. I have much bigger battles to fight.
If I was still 'fat' Hate that word, I would not go to that gym. Imagining the laughing behind my back. Just in poor taste but not upsetting.