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*
start a "diet" (when I say diet I mean eating healthier) and exercise routine with her...i agree with the other ladies, compliment her on opposite clothing styles but also a lotttt of couples workout together and go on healthy "diets" (I hate that word) together. you could tell her you feel like you're getting out of shape and/or you're kind of feeling insecure so you want to go to the gym and would like to start going together that y'all could help keep each other in line and be each others' motivation;) y'all could even make it a competition "who can lose __lbs before the other" or "who can go the longest without ____ (some food or drink that causes y'all to gain weight)" lol actually, a lot of the couples I've known that have done this, the females have been the winners XD it's funny because the guys get so p*ssed off! XD
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."
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..."