Sumo Wrestler Once Known as 'World's Heaviest Boy' Dead at 21

A Russian sumo wrestler who used to be known as the "world's heaviest boy" has reportedly died at a very young age ... this after years of concern about his extreme weight.

Dzhambulat Khatokhov recently passed away, per Betal Gubzhev -- president of Sumo and Mass Wrestling Federation in Kabardino-Balkaria ... that's where Khatokhov had lived and grew up wrestling. A cause of death is unknown, but word is he'd lately had kidney problems.

The guy was a worldwide phenomenon for a couple reasons -- for one, he competed on the mat, but also other strength challenges (like arm wrestling) and proved to be successful at a young age. Second ... he was absolutely enormous size-wise, especially for his age.

At birth, reports say he only weighed in at around 6 lbs -- but by his first birthday ... he already clocked in at over 28 pounds and continued growing heavier and heavier. By the time he was nine, they say he weighed well over 300 pounds.

Khatokhov became the subject of much speculation, and the focus of several documentaries over the course of his life. Different experts examined him, and most expressed worry his life expectancy would be drastically shorter than normal kids his age.

You might think there was some sort of genetic defect he was dealing with that was causing his excessive weight gain and growth, but according to his mom there was nothing doctors could find to explain it.

She reportedly said this is just how God made her boy, and she, apparently, believed he was fully healthy.

Khatokhov was just 21.