Why are cells relatively small?
Cells are so small, so that they can increase their ratio of surface area to volume.
Surface area is defined as area covered by outside (or surface) membrane which is called as cell membrane.
Volume of cell is defined as space used by inner cellular contents.
Therefore, cells are so small, so that cell membrane can supply the inside contents or organelles with their needs in order to survive.
If the ratio of surface area to size is too small, then mitosis or other processes will not work and cell cannot survive. Therefore, surface area should be larger than the volume of cells for these processes to occur.