Sot Chitalada was an outstanding, long reigning champion at 112lbs and even fought for a title at 108lbs. As a Thai, it's not surprising he weight cut, but his was apparently extreme. His manager, after he had a strained performance, said he wanted Sot to move up to Super Featherweight (130lbs)!
Are you talking about the largest weight cut or the largest cut and regained on the night of the fight? Guys like DLH and RJJ cut loads of weight and didn't regain it and we're massively drained and suffered as a result Where as guys like Pacquaio, In Jin Chi, Scott Harrison used to regain 14-17lbs after the weigh in. Alvarez, Margarito and Clottey used to gain back crazy amounts. I'm sure countless others did the same. I think McClellan was a 180lb guy outside the ring before dehydrating. One theory about his blood clot in the brain is that the dehydration to make weight meant his brain didn't have adequate water to cushion the impact of blows.
Based on body weight percentages, Jeff Fenech at 118 has to be up there. He fought his senior amateur career at 119 and was severely struggling to make 118 as a pro. A KO article stated that even at 126 he routinely gained at least 12lbs after the weigh-in.
I knew a guy in Texas that fought at 129, fighting 6 and 8 round fights. He would be weighing 155, 158 and sparring 8 hard rounds two weeks before the fight , then you would see him at the weigh in looking like a malnourished six year old. Ten days later he would be 158 and sparring hard.
I remember it being said that Charles Atkinson (Chitalada's trainer) would get fighters down to what they thought was their fighting weight, and then demand they lose another 10% of their weight. It was successful, but did seem to lead to, unsurprisingly, relatively short careers.
Chris Eubank lost 19lb in 4 days for the first Watson fight, same-day weigh-in! He stood Watson was on his head for five rounds, then his limbs were gone from dehydration and he still somehow won.
Arturo Gatti had some pretty notorious weight cuts (ask Joey Gamache about that). Miguel Cotto's extreme cutting and subsequent overnight weight gain was a continuing narrative during his time at 140. His last 3 or 4 fights he looked downright sick the week leading up to the fight, and no surprise, he was totally rejuvenated when he belatedly went to 147. Anyone else who followed his career back then will remember what I'm talking about. Erik Morales at 122 was just about the most stretched out of any fighter I've seen. I know Fenech was pretty extreme too, I'm just going off what I saw. I can only imagine the torture Morales put his kidneys through getting down to junior feather.
Jamie McDonnell was reported to have weighed 144lbs on fight night against Inoue. 26lbs may not be the biggest cut/rehydration in absolute terms, but it takes some beating relative to bodyweight.
Hatton grew up in a pub .. So after fights back to the pub for beers and throwing darts .. Ballooned up 30 pounds often I think