400-pound high school running back

[SolidSnake84]

John Krahn is much bigger and taller:

What college landed 7-foot, 440-pound lineman John Krahn? | USA Today High School Sports

In his "peak" he was 7 foot tall and 440 lbs at 17 years old. Incredibly athletic and fast, he was told that he was too "big" to play football at a college level, so he started a diet, got down to 390 lbs, with the goal of reaching 360 for his playing weight:

7-footer John Krahn, the largest man in football, finds new home at Lincoln University (Mo.) | USA Today High School Sports

I still don't think that was the biggest guy ever in football. This is the biggest guy i could find: 6 foot 11, 500 pounds, his nickname is "House"

https://www.youtube.com/watch?v=HmqxS4i2ul8