How can some eat lots and never gain weight?

If a slim person eats little and often, maybe it gives an impression to others that they always seem to be eating, thus gives the illusion that they must be consuming lots of calories?

If the snacks a slim person eats are generally low in calories, the total intake might not amount to excess energy intake.

Another possibility that could be at work here is to do with activity levels. Slimmer people could be more energetic because they are lighter and require less effort to move around. This could be helping them to move about enough during an average day to maintain or cause a negative energy balance.

In comparison to a slim person, a heavier individual has the potential to burn more calories during exercise, due to the extra effort to more the weight, but the high effort may be slightly uncomfortable for some and generally they may not move around as much as a thinner person. This would result in a heavier person expending less total energy so they could end up struggling to lose weight.

The portion size of food can mask the real intake of food. Some overweight people could be thinking they don’t eat much because they only have 2 or 3 meals each day, but if portion size is large enough then total energy consumed can rise dramatically. Just one Chinese take-away meal for example, can be as high as 1500 calories; nearly the total energy intake for an average woman!

Remember the most basic simple fact...

If energy in exceeds energy out, it will end up as stored fat.