People are getting taller. I asked my husband why, but I quit listening about half way through his answer.
I used to know the answer to this, but now my tiny brain can't remember it.
I worked on a whole bunch of "KGOY" marketing stuff for dolls and other toys. Kids Getting Older Younger was based partially on the bigger and bigger shoe and clothes sizes, as well as heights, in subsequent generations of children in the 20th century.
It's not as simple as food or health care.