Watch everything she's watched (including commercials... which are rife with 'Does this make me look fat?', 'slimfast', 'weight watchers', etc on children's channels (one has to assume, marketing towards their mothers)... and I'm sure you'll find dozens of references.
Pop on over to the school... and you'll find dozens more.
Just listen in daily life (family, friends, radio, even standing in front of the mirror choosing and outfit as you smooth certain things, don't like the cut that day and try another) and there are scores of references to being fat.
Where she's getting it from is horses & barns at this point; it's there.
2 ways I go about it with my own son:
1) Fashion as art. No matter what your body type there are things which will look stunning on you, and things which will look dreadful. ((Disguise &/or spy shows is one way I go about it with my son... watching the same actor in the same body look like 6 entirely different people just from their clothing/ walk/ stance/ facial expression can be quite a fun thing to do)).
2) Cultural differences. In THIS country being called fat is an insult. In some countries, it is the height of beauty. Looking at different forms of beauty across the globe (and really, truly finding beautiful things in what someone else finds beautiful) can be fascinating. National Geographic is a fantastic tool. From the 'outlandish' (women who brace their necks may look freakish... but look closely... look at the patterns and fine detailing in the metal, the choice of colors and layers to create an overall effect) to the opposite of ours (immensely fat women in certain areas of africa -I can't say specific countries, as beauty standards vary by regions within countries, and stay the same across political boundaries in other... the incredible smoothness and perfect proportion of a rounded shoulder, the ... esp. in contrast to the sick/starving/dying) to the color & cut choices of western culture but different nations (I cringe at E European male casual colors until I shake myself loose, and have to shift my brain into gear not to think women's "modest & feminine" in the balkans as "trampy"... it's NOT trampy, it's just a different version of demure than I'm used to seeing HERE. Just like women in America aren't all tramps because we don't cover our hair.)...
ANYHOW, my son and I have periodically gone over many different cultural things... (it actually started when he was about 4 because he was trying to complement a fat woman on her good luck, after spending a great deal of time with family friend who was trying very very hard to just put ON 30lbs before a wedding)... so when something pops up, I still occasionally (at age 8) have to ask him what culture we're in right now.