The size of a cell has a lot to do with the rate at which nutrients go in and out of it. For a cell to remain functional, it must constantly get enough nutrients and other essential gases. To achieve this, cells must remain very small in order to get gases and nutrients into and out of them.
This shows that as a cell gets larger, it reduces the rate at which nutrients move in and out of it. The reason for this is that the surface area of a cell does not increase at the same rate as its volume.
The implication is that as a cell gets bigger, its smaller surface area won't allow nutrients in at the right quantity the cell needs them. On the other hand, a small cell has the right surface area that allows it to get enough nutrients. However, this explains why most organisms don't have huge cells.