Question

If a large surface area is helpful to cells, why do cells not grow to be very large?

Answer 1

Because a cell needs to take in a certain amount of energy & nutrients, and remove a certain amount of waste, in order to maintain itself. 

Answer 2

The surface area to the volume ratio gets smaller as the cell gets larger. Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume.
   ~Hope it helps