Liao: Well one of the things that we noticed is that human ecological footprints are partly correlated with size. Each kilogram of body mass requires a certain amount of food and nutrients and so, other things being equal, the larger person is the more food and energy they are going to soak up over the course of a lifetime. There are also other, less obvious ways in which larger people consume more energy than smaller people---for example a car uses more fuel per mile to carry a heavier person, more fabric is needed to clothe larger people, and heavier people...