Sunspots are caused by the Sun's magnetic field, a complicated, churning field that is always evolving. Some of the changes are slow, like gradual sunspot changes, and some are fast, like when the field changes due to solar flares. When sunspots look like they are shrinking or growing over time, that means that the magnetic field is changing. If the magnetic field becomes more concentrated and complex, then we may see bigger or more complex sunspots. If the magnetic field becomes simpler or weakens, then the sunspot could gradually weaken and die out.