No, it's not an optical illusion, the moon on the left is actually bigger, because the moon is currently in perigee, it's closer to us. The orbit of the moon is not a circle around the Earth, but more like an oval. This time of the year, the moon is 30,000 miles closer to us so it appears larger in the night sky. In fact, it's 14% larger and 30% brighter than any of the other full moons of the year.
According to the Farmer's Almanac, the October moon is the Hunter's Moon. Back when we hunted for our food, before Publix, the bright moon made it easier to hunt in the fields at night. Also, fall crops were more easily harvested at night. Even though we don't do much hunting or harvesting anymore, it's still cool to look up at the moon on a clear fall night. Enjoy.