Lee McAllister says his days at heavyweight are over, admitting he was "crazy" for stepping up seven weight classes to fight Danny Williams.
The Scot created history on Saturday by becoming the first lightweight champion to win a heavyweight title.
Williams, 45, had a four-stone advantage but was floored three times before the towel was throw in during the 10th round in Aberdeen.
"I wanted to prove a point," said 35-year-old McAllister.
He lifted the WBU belt at the Beach Ballroom against the 6ft 3in Williams, the two-time British heavyweight champion who stopped Mike Tyson in 2004.
Williams had previously gone toe-to-toe with the likes of Vitali Klitschko, Dereck Chisora and Audley Harrison, and had won his last three fights by first-round knockout.
McAllister's natural weight is the 10-and-a-half-stone welterweight limit, but the unlikely match-up with Williams came after he stepped in as a late replacement to fight heavyweight Lee Kellet, who he beat in four rounds in April.
'I was very comfortable, I beat him easily'
He told BBC Scotland: "I stated I'd love to share a ring with a heavyweight like Danny Williams, a legend.
"But he got wind of it and came back with 'If I was to fight Lee I'd smash him in three rounds. I'd kill him, it's bad for his health, and I'd destroy him'. That got my back up a bit.
"I was very comfortable, I boxed well. I beat Danny, I beat him easily and no-one expected me to be able to that.
"The way my body is feeling after that 10 rounds, it's a totally different kettle of fish from fighting someone at 10 stone and fighting someone at 18-and-a-half stone. It's crazy, it is actually crazy.
"I'm done at heavyweight, I'm moving back down to my proper weight which is welter or maybe super-welter."
McAllister has previously won titles at lightweight, light-welterweight and super-welterweight, but says it is "overwhelming" to have beaten Williams.
"It's a massive achievement for myself, for Aberdeen and for Scotland," he added.
"But the whole thing was just to prove doubters wrong, and prove to people that if you believe you can achieve – it doesn't make a difference if you're five stone or 20 stone. If you believe in something you can do it.
"To be the first person ever on the planet to come from the lightweight world champion, step up to light welterweight world champion, welterweight world champion, and then step up seven weights to fight a heavyweight and be heavyweight world champion, is an awesome achievement."