1. Rarity said in the begnning of the episode that baby unicorns can have outbursts of magic. 2. A baby is much MUCH lighter than a full grown unicorn, therefore it requires less magic. 3. Why didn't twilight levitate herself when she was about to fall down a cliff? Or rarity falling at the sonic rainboom episode?
soo... basically, magic is different for everypony, and its just the neccesary for their special talent, and donuts... well, lifting magic might be just for that, not for biggr things, it would explain everything