Jay Deas, the head trainer for WBC heavyweight world champion Deontay Wilder, admits his fighter was unexpectedly too light for the bout with Tyson Fury in December 2018.
The rematch takes place on February 22 at the MGM Grand in Las Vegas.
Their first meeting ended in a controversial twelve round split draw, at Staples Center in Los Angeles.
This time around, his head trainer wants him to be bigger and stronger for the rematch.
"One of the problems with the first fight is that Deontay was so excited that he wasn't eating right. He came in unusually light," Deas told Sky Sports.
"He was 209lbs on fight night - by the time he knocked Fury down in the last round, he would have been a 200lbs, essentially a cruiserweight. We don't want that. We want Deontay to be bigger and stronger."
When Fury returned from a near-three year hiatus, Ben Davison was his trainer.
Last month, Fury shocked a lot of people when he dropped Davison and replaced him with Sugarhill Steward.
Deas views that move as an advantage for Wilder.
"Any time a trainer and a fighter who have done such wonderful things together split, it has to be an advantage for the next opponent. Chemistry is not an overnight thing. Ben was not just Tyson's trainer - he was a brother, confidant, friend, therapist. Those kinds of guys don't come along every day," Deas said.
"I think a lot of Ben - for someone as young as he is, he is incredibly knowledgeable and strategic. He picks up on intricacies and nuances better than people who have been in this game for decades. Ben's potential is unlimited. I enjoyed the challenge [against Ben]. I know Ben saw every tiny detail, so I had to do the same.
"[Steward is] a good trainer who comes from good pedigree from the Kronk gym. But this is a big fight that he's stepping into."