Often you find that sunspots sitting by themselves are quite round,
whereas sunspots in big groups have more complicated shapes. Sunspots
are where loops of strongly magnetised gas are breaking through the
surface of the Sun from deep below - the sunspot is a slice through
such a loop. If one loop comes up by itself then it can be quite
simple, but if a complicated bundle of loops comes up then the sunspot
shapes is complicated. Everything depends on what the magnetic field is
doing deep down inside the Sun, and unfortunately we don't understand
that, yet!