"most of the time".

Is there any evidence that 50%+ of overweight people are that way because they consume too little or just enough?

Or, instead, would you find just the opposite? And that overweight people grossly underestimate the number of calories they consume...

It very much is, it is complex but ultimately it is just balancing an energy system. If calories in > calories out then you gain weight etc.

What is the reason for their fatness then?