why?
how do you ‘XL’ a pair of jeans without making a new pair?
what about gas masks?
what about etc?
no, i dont agree. You have to make new sets of clothing.
XL clothing is more than just bigger sizing. its accomedating your unique anatomy you mutie. 
each body part, each new limb jutting out at an impercise location, means even that Id argue against 2 XL mutants sharimg fitted clothing