The term "XL"/"Extra Large" is used to make jokes upon a person who creates/makes fake stories about their "XL(extra large) penis".(1)
The "XL"/"Extra Large" is used at any time a descriptive word in a converstaion is needed(2).
XL originated from a person who believes themself to be a higher/better human being than the rest of civilisation.
(1) "Then she went through my drawers and found an empty box of XL/Xtra Large CONDOMS, can u belive it? An EMPTY box of XL/ Extra Large CONDOMS!!!"
Person 1- These very big crisps!!
person 2- You mean, very XL/extra large crisps!!!
Person 1- Seen this poster, its XL/Extra Large!!!