The human body is a complex adaptive system evolved over a billion years. How much a person weighs is going to be determined by many factors (genetic, mental, social, etc.) that vary in strength from person to person and change for each person over time. No simple solution that everyone can use to get to their "ideal" weight is likely. Scott Alexander has some interesting thoughts about it[1].
Edit: It seems to me that if exercise helps in keeping a low weight, it is likely that it is from some long term feed back loop. Having a bunch of extra fat is good and can get you through the famine when skinny people will die. But, if your life depends on running lot (hunting down antelope, running from hyenas, etc.) being heavy will be a disadvantage. The body tries to balance these out in some way (along with hundred of other things). A similar trade off happens in muscle formation. The body will only build up expensive to maintain muscle over a long period of time of pushing them near their existing limit.
Edit: It seems to me that if exercise helps in keeping a low weight, it is likely that it is from some long term feed back loop. Having a bunch of extra fat is good and can get you through the famine when skinny people will die. But, if your life depends on running lot (hunting down antelope, running from hyenas, etc.) being heavy will be a disadvantage. The body tries to balance these out in some way (along with hundred of other things). A similar trade off happens in muscle formation. The body will only build up expensive to maintain muscle over a long period of time of pushing them near their existing limit.
http://slatestarcodex.com/2017/04/26/anorexia-and-metabolic-... [1]