Because it isn't just the diet that makes sumo big. You know this one Gentle Asker. ^__^
Sumo eat, then they *sleep*. That is what makes them fat.
Not to mention, when they are physically active, it's with grappling and other power-based exercise. Nobody ever does any cardio, and even the lighter weight sumoka (below the 350 pound range) are built like power-lifters, thick in the middle, because of it.
Actually, that phrase, "thick in the middle" sums it up. ^__^ Nobody in a Western culture likes that idea. When they think "bigger" they usually think in a body-building sense of huge chest, huge limbs....but not the torso.
But yeah. Details. ^__^ All *good* Best Answers are in the details.
Thanks kindly for your time!