Of course her voice was better in KH2 - she didn't laugh.
I donno. Her dress is nice though they do give us too many poses in which she looks too slutty for words. That, and for some reason her face seems deformed to me. I have no idea why, especially since Namine's sprite doesn't bother me as much. Maybe it's the hair color and texture, I donno.
Thin =\= lacking muscles. Look at him in KH2 Atlantica - he's not buffed up, but he's not just a skinny bag of bones, either. He has muscles, they're just powerful without taking up "space". Look at Tarzan by Disney design - you can't say he's not muscular, but he's 'skinny'. It's just that Sora has a different type of muscle make-up than say Riku. Riku seemingly goes for brute strength while Sora focuses on agility. They're both strong, but their physical builds are different.