Hi Erin,
We don't completely know the answer to that question.
We think sunspots are made up of bundles of magnetic field that rise up from below the Sun's surface. At solar maximum there are more of these bundles and you tend to get bigger sunspots and sunspot groups. I don't think we know why one sunspot is big and another small, though. This is the sort of thing we would really like to understand; it would help us predict solar activity.

Terry