Also I'm pretty sure on the netori path we're probably gonna be stealing at least a few girls from the futa upperclassmen. Make a solid statement to them to stay the fuck away from Milly's women. Milly having a bigger dick serves pretty well for that.
Plus part of the lore is she's the ideal futa. So horny that the sex suppressants barely worked on her, has a lower refractory period, longer endurance, higher sex drive, it'd be weird if she didn't have a bigger dick to go along with the rest of her sexual superiority.