Raging obesity is not such a huge issue in Japan compared to countries like the USA or Russia (though I hear McDonald's is gaining foothold). Ironically enough, they nevertheless have a strange obsession with Sumo Wrestling over there – two incredibly fat guys basically turning into a big mass of jelly.
Aa Harimanada apparently began its life as a comic book which later managed to take the leap into an animated cartoon and from there into this console game. What the plot is exactly, I can't tell you as all these publications (including the game) are available in Japanese only (a language I don't speak at all). It probably doesn't help that I'm also no expert at the rules of Sumo Wrestling (still an understatement). What I do know about this sport is that it is basically about pushing the opponent out of a confined circle.
This also appears in this game: whoever is out is beaten. There is also a lot of grappling between the hulking masses of flesh. The similarities end there, though. At least I doubt that any real Sumo Wrestler of the real world has ever made a jump, let alone one like this. Which is to say that this one uses pretty much a standard Street Fighter inspired gameplay, complete with acrobatic stunts, punching and kicking.
Now, that actually does sound more exciting than an accurate simulation of the sport, doesn't it? Well, probably, but on the other hand, this pits it against huge competition in this extremely popular and crowded genre at the time. And, boy does Aa look bad in this competition! Where to start? With the almost completely interchangeable opponents? With the severely limited set of moves? With the rather unresponsive controls? With the sparingly framed animations?
Whether this has anything to do with the comic book, as I said, I cannot say. Between the fights, you basically get the regular taunting (I assume…), but that's as far as the "plot" goes. One can only hope that the material which inspired it has got a little more meat to them…