The only way to tell whether/not it's enlarging will be to use serial photography....however, in this case, while I would be taking pictures at a set schedule, I would also have the cat seen immediately.
Is it a scab? If its the result of a minor wound it should get better soon, if it doesn't then a vet visit is in order as it could become infected. And of course there is a very small chance that it cold be cancerous - but its premature to panic about that, most likely it is a small wound. You could probably give it a week unless it gets worse, and if worse see a vet sooner, and if not resolved in a week or so then see a vet still.
