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