Because I like large, oversized toilets. 
I'm big and my name is John.
I didn’t even know this was a thing until I did a little searching:
https://mybesttoilet.com/best-oversized-toilets/
I've known john was an informal word for toilet since I was a little kid. But now I'm curious if that's just because I heard my grandma say it a few times, or if other people know this as well 😅
I've known that for as long as I can remember, in part because the best-known purveyor of portable toilets in the DC area is Don's Johns (to the point where when I was growing up, you didn't call it a port-a-potty or an outhouse, you called it a "Don's Johns," even if the purveyor was competitor Jiffy John).
But when I was in high school I got in the habit of referring to it as the head because one of my teachers, the late John Donohue, didn't like it when people referred to "the john."