I don't know the details behind it but I can imagine a number of factors. - Inability of us to efficiently consume large quantities of food so we pass more cal

I don't know the details behind it but I can imagine a number of factors. - Inability of us to efficiently consume large quantities of food so we pass more calories than we would for smaller levels. - Changes to metabolism to adjust. I think at the end of the day it is still calories in/calories out but you may be taking in less than go in your mouth and may be using more than you usually would.