Oh honey. You were tricked by girl speak.
There's not a lot you can do except apologise (even though she put words in your mouth) and tell her you love her as she is, and you don't like skinny minnies etc. I don't think 12/14 is fat, unless she's 3 foot tall. You get me.
If she asks this kind of thing again, you must always say " I love you as you are, but if you want to change, then I will support you".
Oh, and "Do I look fat in this/does my bum look big in this?" Answer is NO, or "No, but you look lovlier in XYZ dress, try that one."
ANd "How do I look" "Lovely" "Gorgeous" "Beautiful", NEVER "Fine!"