The way I thought about it, you have to roll around the larger one and you have to roll around yourself (+1). So if they are two same size circles, it's 1 for the other and 1 for yourself. If the other one has 3 times bigger radius, it is 3 for the bigger one and 1 for yourself. Etc.
Not sure if it is mathematically accurate but it seemed to make sense to me.
Not sure if it is mathematically accurate but it seemed to make sense to me.