Cells remain small because it allows them to most efficiently utilize nutrients, excrete wastes and replicate. Their smaller size allows them to maximize cellular energy.
Being small gives cells a better surface area to volume ratio. This makes it easier for cells to utilize nutrients and excrete wastes. The smaller the cell, the less time it takes to replicate by a process called binary fission.
Cells minimize the distance between their nucleus and organelles by staying small. Being small allows them to maximize intracellular communication and provides ideal conditions for diffusion. This allows for better absorption of oxygen and nutrients by the cell.
If cells were larger, the distance between their nucleus and organelles would increase. This would make it hard for material to diffuse across the membrane quickly enough to accommodate the increased cellular volume. Therefore, cells must divide into smaller cells to maintain a proficient rate of intracellular communication.
Cells stop growing in size because larger cells would require more energy to produce enough proteins to maintain the cells. Being small makes a cell more efficient in absorbing oxygen and nutrients. This allows waste to be more efficiently excreted and ensures the cell can maximize its intracellular functions.