Small cells function more effectively, because as cells become larger the surface area to volume ratio:

Small cells have larger surface area to volume ratios than big cells and this makes them function more effectively.

It is the surface area of an object relative to its volume.

Cells with large surface area to volume ratios are able to effectively carry out important processes because of their small sizes.

On the other hand, big cells have reduced surface area to volume ratios and thus, find it difficult to move materials by diffusion across their membranes.

More on surface area to volume ratios can be found here: link