Toriyama talks a bit about this change in his his
Daizenshuu 1 interview,
Q: Since this volume is a collection of DragonBall illustrations, I thought that I'd center my questions on your pictures. Do you feel that you made any deliberate changes in your pictures between DragonBall and the earlier-serialized Dr. Slump?
A: Yeah, I don't really like to go for the same sort of pattern, so I change the pictures to suit the story. Because of this, even now if you tell me to draw with a Dr. Slump-esque touch, sure enough I can draw like that. I just use round lines.
When I got into the second half of DragonBall, I had already become more interested in thinking up the story then in drawing the pictures. Then I started to not place much emphasis on the pictures. The battles became intense, and I gradually came to switch to more simple lines.
At any rate, I despise doing the same thing. I'm fundamentally perverse that way. I got postcards from readers saying "Compared to the old days, it's really square now. It was better the way it used to be.", so I thought I'd go make it even more square. (laughs)
However, at the start I didn't really have much confidence in my battle scenes. I hadn't really drawn what you might call motion pictures before. That's because I started out in illustrations. It was really hard to do that first Tenkaichi Budoukai.
Elsewhere in the interview though, he does credit anime (though not the DB anime) with inspiring him to change his style a bit:
Q: The way that you paint colors has also changed quite a bit.
A: Yeah. For example, in the old days I shaded off the light portions on hair, but shading it off took a lot of time. When I made an anime called Kosuke-sama Rikimaru-sama (screened at the '89 Jump Anime Carnival, and later become a Jump Video), I looked at the pictures of the animator Toyoo Ashida, and I thought that the anime-style way of applying light and shadow wasn't bad. I made my pictures like that from then on.
Later though, Toriyama says that he generally doesn’t consciously try and change his style, it just sort of happens anyway:
Q: In this book, you can see all the pictures in chronological order. Your particular touch and the way that you paint have changed over the years.
A: But I didn't really do this consciously. It basically changes without me knowing. But if I look at a tankoubon or something from a year back, I think "Ugh! This was done poorly."
Q: After a year?
A: Yeah. I think "Ah, I guess I've evolved a lot." (laughs) Even with pictures from a little while back, I still think it looks weird. For example, with even a manuscript from about half a year ago, I feel that the design is weird. With color manuscripts as well, after I've painted it I usually think "Ah, I messed up."
In
Shenlong Times #1, Toriyama’s second editor, Kondou Yuu, discusses the change in Toriyama’s style from round to sharp lines a bit, reinforcing Toriyama’s statement that he made the change in order to better convey battles:
YK: That's when it started to change from what it had been before. When it started serializing, the story was comical, so the lines were round. Then, with the Tenkaichi Budoukai, it switched to a fighting theme, so after that the fighting had to escalate. But when it comes to that, round lines don't convey a sense speed or intensity. So once he realized this, midway through the manga the round lines started to become square and sharp. When that happened, I said to Toriyama-san "Aren't the lines a little harsh now?", and he said "No, if I don't do it like this then there's no sense of speed."
So basically, it seems that if Toriyama was imitating the anime style when it came to switching over to his sharp line style, it wasn’t a conscious or intentional choice. It’s possible that he was unconsciously inspired by the anime style though; we know he did watch the anime every week, and he wasn’t above taking drawing cues from anime.