what a strange question.
data is stored on platters inside the hard drive. kind of like a double sided dvd. you can only fit so much data on a platter. even if you manage to make each bit one molecule in size, a platter could still only hold so much. hard drives have anywhere from 1-5 platters inside, they cant fit more without making the hard drive thicker. they dont make hard drives thicker because they have to be a certain size to fit properly in a case.