Most shows that do feature children of different size have the caricature of the fat child or animal (think the gluttonous and out-of-shape cat on the latest Shrek) who is funny, lazy, silly, stupid, mean, or all of the above.
I wrote awhile ago about a show called Caillou for the younger set, and I've recently let M watch Lilo and Stitch (Netflix, commercial-free electronic babysitter while my childcare issues continue...)
Lilo is active, has friends of all sizes who are active. There is no mention of "being healthy" or using someone fat as a cautionary case.
As a bonus, none of the little girls seem sexualized. I have to say, I don't like all the teasing, the "mean-girl" language about how she is "weird," but overall it's one of the least offensive shows out there.
Are there books, shows etc that you like in terms of body-diversity? (I'd be curious if there are any ethnically diverse shows out there in any real way...)