Hi! This has to do with how strong the magnetic fields are in the spots -- the reason the sunspot is dark, is that it is a region of strong magnetic field where heating is suppressed so it is cooler than the surrounding area and so darker. Generally speaking, big sunspots have strong magnetic field. Also, sunspots tend to come in pairs, and the one in front (in the direction of the sun's rotation) tends to be one big one, whereas the spot behind tends to be fragmented. cheers, Sarah