To answer your question, it just depend on your metabolism. If you have a HIGH metabolism you can eat and eat and eat and not gain. :) Or skinny people do the gag trick, where they eat and then throw it all up seconds after, they will pretend as if they have to use the bathroom and then while their in their gag it all back up. <-- it is actually pretty nasty >.<
- Hope this helps!
Simple answer? .... They don't.
People who are slim don't have some magic capacity to defy the laws of physics.
In order to put on weight, you need to eat over your maintenance caloric level (the amount of calories your body needs to maintain it's current weight).
So if you were 150 pounds, that would be 150x15 = 2,250 calories/day.
To gain 1 pound a week, you would need to eat 500 calories over this number each day...that would be 3,500 calories extra each week which is the amount of calories needed to add on 1 pound.
Now when you see these skinny people "getting away" with eating "all this food"...here's what's likely happening:
1. They don't ALWAYS eat like that.
It is possible that hypothetical 150 pound person eats LESS than the 2,250 calories they need to maintain their weight 6 out of 7 days of the week...but then on the 7th day they go out with YOU and GORGE on food, thus evening everything out.
2. They are very physically active.
They could also be offsetting all that food you see them eating by doing a lot of cardio. If they run every day or their job requires a lot of walking around, they could be burning off a lot more calories each day then you are, and thus get away with eating more.
3. They AREN'T eating as much as you think.
Perhaps they didn't eat anything all day and then went out with you and ate their total day's calories in that 1 meal...
perhaps the food they're eating isn't very calorically dense.
For example 1 pound of broccoli is only 154 calories, while 1 pound of pizza is about 1,200.
Regardless...the reason is going to be one of the above listed reasons...there is NO WAY to cheat physics on this!
Calories in vs calories out...More info on all of this here.