“Fatso”, by The Story. The album is Angel in the House. Chorus:

Because someone will adore me when my ribs show clearly
and I’m thin even when I sit down
Someone will admire my gorgeous arms and legs
when I’m only one hundred pounds

Yes, it’s satire, or some such technique. Full lyrics, and it’s available on iTunes. The Story is part of the genre I call “Lilith Fair,” except I’m not sure anyone under 25 or so would get what that means.