Sumo Wrestling was a scam that Eddy came up with in "One Size Fits Ed." He was inspired when he saw Ed pretending to be a sumo wrestler, and initially planned to make Ed the wrestler, but Ed turned him down. Soon, though, Eddy remembered that Jimmy wanted to be famous, and roped Jimmy into the scam.
Eddy's plan had three stages: The first stage was to make Jimmy gigantic, as sumo wrestlers are, and they accomplished this by having Jimmy eat his stash of Peaches & Cream. With Jimmy fat, they then trained him by having him wrestle Ed. When Jimmy won, Eddy was ready to start sumo wrestling, but he was stopped when Edd pointed out that sumo wrestling is really only revered in Japanese culture. For this reason, Eddy had to execute a third stage, which was to figure out a way to get Jimmy to Japan, and this is where his plan fell apart. Initially, he tried simply mailing Jimmy to Japan, but Jimmy's now-immense proportions inevitably broke the mailbox, and Edd points out that even if it hadn't broken, postage would've easily cost up to $200. His second idea was to try and use a sort of streetlight slingshot to send Jimmy off, but this also failed when Jimmy slipped off the slingshot in midair and plummeted to the ground, crushing Ed and Eddy underneath him.
After they were crushed under Jimmy, Ed and Eddy were next seen in Edd's bedroom, being tended to by Edd. They were in full-body casts and were largely unable to move. Jimmy didn't fare much better, as he was seen exercising to try and work off the extra pounds he gained under Eddy's tenure with Sarah as his coach.