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!