I can go on forever about how exercise has physically challenged and changed me. But above all, spiritually, this has been a lesson of evolving, facing my fears and feeling deserving to (as my sister says) be proud of and own the body that houses my soul, that takes you to the next day, the next dimension.
The longer we believe only skinny, white, affluent girls suffer from eating disorders, the more we isolate an entire community of not-skinny, not-white, not-rich, not-so-young, decidedly-not-female human beings, who suffer, not only with the soul-sucking burden that is an eating disorder, but with the belief they can't possible "have" what's killing them.
Talk to your daughters about how there is no virtue in striving for being a 0 in any realm of life. See if you can get your daughters to see this size as ludicrous. Use humor if you have to. Most importantly, let your daughters know that you are aware of these sizes and that they are unacceptable to you.
We tend to see eating disorders, our general cultural anxiety about food and weight and skyrocketing obesity rates as separate problems with separate solutions. Instead, we need to recognize that they all stem from the same root. Obesity isn't the opposite of anorexia (or bulimia or disordered eating or just distorted thinking about food). It's its twin.