Giving Goku a massive growth spurt wasn’t just to show how much time had passed – it made things a little easier for Toriyama. Goku’s small stature made the series’ battle scenes more difficult and as the story became more focused on tournament arcs, making Goku adult-sized was a necessity. Toriyama revealed this decision in 2015, while discussing the series for his 30th anniversary. He said:
“I had a lot of reluctance about it at the time. Apparently, in shōnen manga, changing what the main character looked like was a big no-no, but I didn’t care. His head-to-body ratio made tough fights, so I said that if the show was going to start focusing more on battles, I had to make him an adult. But it really shocked them: “The show has finally become popular, and now you want to go change everything!” That’s the kind of reaction I got.”
It almost seemed like that reaction should have been discouraging, but Toriyama wasn’t fazed. You know that phrase, “it’s easier to ask for forgiveness than permission”? To hell with the writing, Toriyama had already made up his mind:
“I first drew a sketch of adult Goku and sent it to the editorial staff for their comments. But then I started drawing the draft before I even heard from them. (laughs ) By the time I sent the draft to the editorial staff, there was no time for major revisions, so they were just like “if you’re so determined to do this, then fine…”
In the end, everything worked out for the best. Aging the characters made a lot of sense for the future of the story and pretty soon massive time jumps became the norm.