It is not just different body. Try to fees on pure sugar+vitamins (exact correct amount of calories) - you will be hungry and disfunctional and possibly gaining weight.

Starving people loose weight. They also develop host of problems, both psychological and physical. And our bodies have mechanisms to force is to eat by instinct to avoid that. When those mechanisms fail, we loose weight while being too tired to work, by gaining weight the first day after diet, by being dumb etc.

But beyond that, sugar is not same as same amount of calories from protein.