The harvest moon isn't bigger or oranger. It rises unusually early for several nights running, and that's the whole trick.

Around the September equinox the Moon's path meets the horizon at a shallow angle, so the usual 50-minute delay shrinks to as little as 20-25 minutes at mid-northern latitudes. Farmers got several consecutive evenings of bright light right after sunset, which is exactly when you want it if you're bringing in a crop by hand.