with the efficiency that only youth can harness

But then, the truth was never really the point. Thin women don’t tell their fat friends ‘You’re not fat’ because they’re confused about the dictionary definition of the word, or their eyes are broken, or they were raised on planets where size 24 is the average for women. They don’t say it because it’s the truth. They say it because fat does not mean just fat in this culture. It can also mean any or all of the following:

Ugly
Unhealthy
Smelly
Lazy
Ignorant
Undisciplined
Unlovable
Burdensome
Embarrassing
Unfashionable
Mean
Angry
Socially inept
Just plain icky

So when they say ‘You’re not fat,’ what they really mean is ‘You’re not a dozen nasty things I associate with the word fat.’ The size of your body is not what’s in question; a tape measure or a mirror could solve that dispute. What’s in question is your goodness, your lovability, your intelligence, your kindness, your attractiveness. And your friends, not surprisingly, are inclined to believe you get high marks in all those categories. Ergo, you couldn’t possibly be fat.