What makes sumo wrestling so popular in Japan?

Every nation has a sport that baffles outsiders even as it drives locals into a fervor. In America, our version of "football" usurped baseball as the national pastime, but to the world at large, it's a weird form of rugby with an ill-fitting name. In Japan, sumo wrestling is a sport that seems incapable of gaining traction in the West. The aesthetics of sumo are the primary factor that limits the sport's popularity in the States, but there's more to sumo than meets the eye. For starters, what are the rules, and why are all the athletes so portly? 

I hope the video by Phanny Packster embedded above can answer some of your questions. You may not walk away from the video a card-carrying sumo fan, but at the very least, you'll understand some of the sport's allure.