Perhaps he meant 'phat' (goood), but you heard it as 'fat' (bad)!

But, either way, what an incredibly crass thing to say to someone.

I really don't understand men who think it's perfectly fine to just come up to a woman and say anything they want to her, whether they consider it complimentary or not. It's honestly like some men think women just exist for their eyes and pleasure, like we couldn't possibly have feelings about anything.

Sorry he said that to you.