Increasing the size of the cube-shaped cell increases the volume. However, the surface area-to-volume ratio decreases. This means that the simple diffusion would take more time distribute nutrients across the cell (and even in the eliminate waste). It would need bulk transport such as vesicle transport, otherwise cell activities would slow down.
Explanation: You're supposed to have numerically more smaller units than bigger units, since a lot of these smaller units are combined to create a single larger unit.
