That just kicks the can forward one step. What parameters control the optimal size of a given cell?
there is likely evolutionary pressure against large cell size (selfish genes; larger cell takes energy away from replication, provides more opportunity for infiltration by other genes, fewer gene backups in other cells, etc) while occupying a niche puts pressure to be a certain size. it lands somewhere in the middle.
Why are things the way they are? Because it works better. Simple, really. :D