Changing the size of a pupil

Can anybody tell me how I can scale the size of the pupils in an animation? The pupils are a texture on a common eye ball. I don’t need an exact explanation, just a little hint.

I personally would proceed with a shape (that can be driven) increasing or decreasing the size of the area.

Select the pupil’s vertices in edit mode, Press Ctrl H, this will create a hook to control those vertices you can animate the scale of the empty.