Usually trends can be traced back not to a single individual, but usually multiple people doing something different around the same time. That isn't the case with huge boobs in manga, if you were to beliege Gantz and Inuyashiki author Hiroya Oku.
"You may not believe it, but when I debuted in a manga magazine, no one was drawing big breasts. Even in youth magazines. There weren't even a lot of erotic manga. That's why the seniors around me called me a pervert just for drawing big breasts. I never thought we'd see an era where big breasts are the default even for boys' magazines..."
Long before starting Gantz in 2000, Oku's career began in 1988 with a manga called Hen. I'm sure the world of manga had seen plenty of big breasts around that point, but Oku was brave enough to flaunt it on volume covers.
He continued on Twitter:
"I think Reiko from Kochikame probably didn't have big breasts when I debuted. After all, I was reading the manga wondering why they didn't draw big breasts, so I'm sure I'll remember if they were big near the end. Because I was reading it."
A follower tested his hypothesis by replying with a panel of Reiko from Kochikame before Oku's debut:
To which Oku replied with a panel of the same character from the same manga after he had started Hen:
Hard to argue with facts like that. What do you guys think? Did Oku really popularize big breasts in manga? Leave your thoughts below!