I'm opposite of complaining, I'm with Brian May and Sir Mix-A-Lot. Agree with you completely -- Steph should definitely be a normal kid, especially compared to sinewy, hyper-defined Cass.
I shouldn't be noticing Steph's tush at all though. I remember when li'l Steph was zooming around in her soooperman cape. *snif* They grow up so fast!