A bigger rotor has more leverage than a smaller rotor. Think about stopping the rotor with your fingers (carefully, don’t try this at home), if you could squeeze a large rotor at the outside edge it would be easier and if you stopped it at the hub you need more effort on the hub sized rotor, it is the same as spinning a wheel up. If you spin from the hub you need more effort to get it going. You can probably tell I’m not a teacher.
Aimed at Giddy rob^^^^