Vanity sizing in ladies clothes is, (I think), widely acknowledged as a real thing. A size 12 is now bigger than a size 12 was 10-20 yrs ago.
I've recently noticed it's a "thing" in kids clothing too - being taller is a "good thing".
My DS is 7, and while he is tall he is not "exceptionally" tall. I measure him every Christmas & last Dec he was 135cm which puts him on 50-75th centile but nothing that would make you do a double take if you saw him. Also he is not overweight, I weigh him at the same time too.
He needed new t shirts recently & the t shirts sized 138cm (I think it's reasonable to assume he has grown a couple of cm since Dec) were for AGED 10
Is it just me or is this crazy? A normal 7 yr old wearing 10 yr old clothing