'Toy models' of metabolism and exercise don't work. Something else is going on.

You’ll have often heard the ‘cure’ for obesity is a twofold, easy to implement, behavioural change: just ‘(i) eat less; (ii) exercise more’.

This ‘cure’ is based on a toy model of our energy use: bring the energy you consume in food plus the energy burned through exercise into a negative balance (less food + more activity), and obesity will be ‘cured’ - because you’ll be burning more calories through activity than you take in through eating.