How can I tell her she's too fat for her style?

I used to date this guy who dressed, awful. Like, I was embarrassed to be seen with him in public sometimes.
I didn't want to be too straight forward and hurt his self esteem, so I would buy him clothes.
I bought him shirts that I thought would look good on him, and being that he loved me, he wore it, even if it was so not his style. It honestly looked good on him though. I complimented him. And when we would go out shopping together, Id pick out stuff for him and say, "I bet this would look really good on you", and we'd end up buying it. From shirts, we moved to pants, then shoes, then hair. I gave him a total makeover without him even knowing :) In the end, he realized what clothing looked good on him, and would buy and wear without my opinion anymore. The best part is, he didn't feel like I was controlling or changing his style. And I didnt make him feel ugly. I was just sharing my opinion * winkwink*

You should compliment the clothing you like whenever you see her in it, don't compliment the clothing that makes her look fat. If you make complimenting a habit, she'll wonder why you don't compliment some things. If she asks you why you don't compliment her in some clothes, just say, "You look so sexy in (name a similar item you like)...you look beautiful in that too but (item you like) really brings out."

Also, you could mention something like, "girls look so sexy in (name non-muffin-topping clothing."

If you don't like how she looks, then why do you date her?

Jeez, how about stop referring to it as " fat" that'll get you points buddy *rolls eyes* tell her that the clothes shes wearing don't suit her from someone else's perspective. Then tell her sexy things about her like "well that skirt doesn't do anything for those gorgeous legs of yours what about a skirt like this..."

Go with her shopping and help her pick things out that might better fit.