It really sounds to me like she's just really trying to say she's ready for big girl panties. I have 4 biological kids and 2 steps. Out of those, 3 are girls (2 mine and 1 step). My youngest aren't ready for panties yet (they are both under 2), but my 7 year old girl did the same thing when we were doing the potty training thing. She told me they made her butt look big. I had to laugh.
I think in a society of "ism-s" and "mental awareness" and self professed pyschologist (thank you Dr. Phil and Oprah), we are so quick to judge that our children are developing unhealthy mental growth. I think that sometimes you are allowed (and encouraged) to sit back and have a hardy laugh. Yes, it's true that there's Vogue, RedBook and Fitness mags that adorn the checkout stands at the grocery store. There's all sorts of infuences in the world that try to dictate beauty and some girls fall victim to it (my cousin had a serious ED until she got pregnant and she didn't seem to mind the extra luggage anymore). But hey, the girl is 3 years old, and she is in the midst of toliet training. I personally would give some thought to the idea that she's noticing the difference between what big girls do and she is learning she's ready to be a big girl now. You know...
"Mommy, WOW! I'm a big girl now!" - I love that tune! So, you don't have an ED, and it sounds like she's a happy girl. If she made the comment once, don't stress it. Reassure her that you are proud of her, she's a beatiful girl, and comment on how much she's growing up! I bet she'll glow with pride and feel great!
I think it was a fun, quarky remark, and I laughed when I read it! How adorable!
Alisha