Daxx's Wife here. I took the poll and was surprised to see that I was the 2nd person in my category. I am very thin and petite (38 yo, 5'2", 98 lbs). I am a very active person, high metabolism and it has to be genetics b/c my family is thin (Dad, mom, bro.). I do eat healthy, eat portions until I am full (but not stuffed) and, quite honestly, would opt for dessert over dinner any day of the week. However, I do enjoy potato chips and have to say they are my one weakness. I could eat a whole bag in one sitting and call it dinner. Let's just say that I don't diet but do try to eat healthy and stay active.
I feel fortunate to not have to worry about weight yet. Who knows? Maybe one day, weight will become an issue.
DH, on the other hand, is a bigger guy. He could stand to lose about 30 lbs. but I don't push. I'm sure that his dr. would agree (and probably has said as much). DS has a bit of a tummy but he's going through a growth spurt (he's eating everything again). Once he gains in the height, he will lose in the tummy. I have made him cut down on a lot of snacking, though.
ETA - I don't think I'm extremely thin ... I think I'm just fine. However, everyone else manages to chime in "You're so skinny!" or "No wonder you're cold, you have no meat on your bones". And, no, I'm not afraid to gain weight -- I can eat a pan of brownies and not gain a pound. Not bragging here - but just to let you know what the metabolism is like.