The dependance of how large or fast the unit grows seems to vary. The best way to gorge a Eater of the Dead is to put a Harvester of Sorrow in the same province and let him go at it. I've had a couple Unfettered from 1200-1600 Hp.

The Last Stage (Unfettered) is when you lose control of your Eater of the Dead and he becomes Independant, gorging himself on anyone with him. It is particularly fun to have him go unfettered in an Enemy capital or anywhere that can cause a roadblock. I personally feel that the Unfettered should have a few more abilities (Resistances, higher MR) because it adds a whole 'nother level of fun to kill him without being cheestastic and just orb lightninging him to death ;/