The same reason why Hulk isn't fat. Or Captain America isn't fat. Or any of the other typical super heroes. When they designed the character, the artists designed them that specific way.
She will, believe me, she will. It's bound to happen. Barbie with a big butt......just around the corner. :-)
So that they're easier to hold and play with. Also it would require less materials to make all their clothes and accessories.
Kind of like real life.