Who wears Andrew Christian "small" size?

I ordered a pair of AC swimmin trunks for my nephew , a slender lad  who just celebrated his Eighteenth birthday. Well, judging by the measures give by AC, I  had to  choose XL, extralarge. Now, I wonder, if a slim lad of 18 is Extra-large, who wears small? Not to mention nanofit, microfit, etc. Hobbits? Anorexycs? Anorexyc hobbits?  Quite a mystery... One thing is certain: Yours truly, as far as AC is concerned, belongs to another Universe