I was that kid, too.
Yes, and that's that lady's body and metabolism, and yours is yours. Comparing yourself to "that skinny person" isn't a good thing to do to ourselves, because we don't know what's going on in the rest of his/her life. I think we all have examples we can think of; "mysteriously" thin people on the surface, but it's no big mystery once you know them better. I had a super-skinny teenage coworker who ate a ton of junk and drank soda all day and night. I found out that almost every day, he pocketed what was supposed to be his bus money and walked to and from work and school. He was probably walking 7-8 miles a lot of days.

And hey, I bet *I'm* that mystery skinny person to a lot of coworkers. They see me drinking coffee all day with real cream, and pounding down the bacon and sausages, but they don't realize that that's often all I'll eat all day. When I was single, I basically only ate at home on off days. I had a total bachelor fridge with nothing but booze and condiments.