It’s really quite complicated.
There are a series of mathimatical calculations that I have a sector of the elves work on each year, that takes into account growth, diet, activity, and a number of other factors to determine size. It’s not a perfect science and sometimes we get it wrong. When you get a strange pair of underwear for Christmas from Santa, please don’t be too angry- it’s not for a lack of trying! I have an entire wing of elves dedicated to underwear calculations. I bet not everyone can say that.
I never peek in underwear drawers. I’m far too old for that sort of nonsense. They call me Saint Nicholas for a reason! 😉
As for the why- well it’s simple. Everyone should wear underwear.
Socks… well, the idea is that you wear them enough to stetch them out good. Then hang them up next Christmas for me to fill with goodies for you!