Bellchambers himself said on one of the fox shows this week he was 130 when first approached & had got down to 123. Either way it was ridiculous for these two to engage, mooney’s an idiotInteresting question - so I looked it up. Mooney is listed at 1.95m/99kg and Bellchambers at 2.02m/106kg)
Once you reach 92kg or so you're in the unlimited heavyweight class for amateur and pro boxing. So both qualify.
It's really up to the contenders to match up their fitness, reach etc once they step in the ring. Capability obviously the key.
BTW, Mike Tyson was about half a foot shorter than his opponent when he became undisputed heavyweight champion and 1987. His technique, power (and sharper teeth) kept him there despite fighting opponents much taller and with greater reach.
Mooney was just way out of his class - kept just at reach distance and and failed to keep his gloves up making him an easy target for a stronger guy with a greater reach.