This would be a synonymous question as to, "why do cells not just grow big in size?"
The main reason would as to why cells have to be small, not just during cell reproduction or cell division, but all throughout their cellular life, is that the cell membrane or what they call as outside of the cell, can't sustain and maintain the insides of the cell if ever the cell grows bigger. The cell membrane is responsible in transporting food and oxygen to the insides to promote the growth and ready them for reproduction. The insides grow faster than the outside which is why there is a need to divide.
