[tsoukase]

All life started from procaryotic cells. The step from macromolecules to the first cell cannot be big, otherwise it could not happen spontaneously. On the other side, it must be big enough so that the cell have enough flexibility and functionality to support complex life.

That is, the cell is small enough in order to be produced directly by molecules but large enough in order to be a full living organism (reproduction, metabolism etc). This sweet spot seems to be the cell size we observe.

Later in evolution the size disparity grew because a procaryotic cell swallowed another one to become an eucaryotic and the eucaryotic ones specialized even further.