Some people eat a lot but they don't gain over weight instead of others who eats less but they gain weight very fast , why ?

Metabolism/genetics: Some people have higher metabolisms than others and burn calories faster. Genetics can play a role in the ability to gain and lose weight as well. Some underlying medical conditions such as thyroid disorders can also play a role ion weight gain/loss.

Balance : Weight gain occurs when calorie intake exceeds consumption. If two people eat the same amount, assuming that they both absorb the same, the one who uses less calories than taken in will gain weight. So it will depend on activity and metabolism.