I agree, it isn't how much, it is what we eat. I too suspect the bacteria is a result rather than the cause of the overweight. Still it is interesting that some people in the same family can eat the same things and yet some are overweight and some are not. I always wonder about that a bit. My son and I are heavier and inclined to be heavy. My daughter and husband are rail thin, both of them. It isn't a difference of what we eat since we mostly eat the same things. There is a difference in body structure. My son and I are packed solid with muscle and larger bones as well as extra fat, while hubby and daughter are petite, low muscle and light bones.
Whether other differences exist I don't know. It is interesting though.