Giga Bowser's design was inspired by Sakurai's vision of how Bowser looked before Super Mario 64.

"The old Bowser was scary. In the NES era, characters were more symbolic, so you had room to use your imagination. When I looked at the original Bowser, I saw a frenzied, terrifying monster.

But, advancements in technology have allowed designers to convey characters in greater detail, and in recent games his image has steadily become cuter.

Naturally, this is a current game, so it needs to use his current design. And from there, Giga Bowser (unexpectedly?) happened to come into existence."
Contributed by ZpaceJ0ck0