What made him come to the conclusion that the world was smaller than believed? Was it miscalculations?
I've written about Columbus here a few times. The tl;dr is that he both thought that the Earth was smaller than it is (about 25% smaller); that the Eurasian landmass covered more of the globe than it does; and that "the Indies," that is Indonesia, extended much further east than it actually does. These were minority views but not unknown.
Some previous answers: