Sorry for not elaborating earlier. This post is about people with petite builds, and as such they are going to be on the 18.5-20 end of the BMI. It's interesting, because I am the only petite one in my family, and 110 or lower feels really good to me (I'm 5'3"), but my sister is very trim and athletic, and looks amazing at 145 (she's 5'7"). If I gain weight, it's just fat, so an additional 5-10 lbs is just 5-10% (-ish) of fat, and I'm kinda "skinny-fat", even with such a low BMI.
But numbers aside, I think back to Jeff's calorie density video, where he shows the pic of all the people in the movie theater. Everyone is trim. It's not a matter of society pushing unrealistic photos of models, people are actually just thin. If you look at the sizes of clothing from the 50's, they just aren't the same as today. Marilyn Monroe wore a size 14, but that corresponds to a modern day 6. I wear a size 0 now, but fit into a size 10 from back then.
And, right now my kids are watching the original Fantasia, and when all the musicians got up for intermission, they were all very thin. I think as a society we are habituated to seeing people at an unhealthy weight, so when people fall on the other end of the spectrum, particularly the petite ones, seem out of place.
If we get the food right, everything falls into place.