The 39-year-old said it was a byproduct of training extremely hard over the past three months for a first professional bout which is scheduled for four three-minute rounds. His previous Fight for Life bouts (a draw against Awen Guttenbeil in 2011 and a win over Jarrod McCracken in 2012) have been over two-minute rounds.
Asked about his weight, the former Blues player said: "It's probably the lightest I've been for a long time. I've had to put in the hard work because I've had to do that little bit of extra conditioning to try to get used to the three-minute rounds. A minute doesn't sound like a long time but when you're trying to throw and take punches at the same time it feels like a lifetime."
As head coach of Currie Cup team the Kings, he has had to start his day at 4.30am to get his training in before starting work at 7am.
He admitted the time away from home had been tough on wife Jodene, and children Payton, 10, and Asha, 6, who are scheduled to arrive in New Zealand tonight to watch the fight and stay for a holiday which will last until the New Year.
Spencer said he was well aware of former Kiwis league international Betham's skill as a boxer, but said he was focusing more on himself.
There will be pressure on Betham, too.
He has fought professionally six times, including against Shane Cameron in 2011 over six two-minute rounds, a bout in which he held his own but lost on points.
Betham, 36, said that since then he had focused on his family, business and charity work, to the detriment of his physical condition.
"It's probably the worst shape I've got into in a long time," he said.
"I got into some bad habits ... so for me it was perfect timing and to come up against someone who is a fierce competitor and someone who always comes in good shape."
Betham has dropped 6kg, going from 19 per cent body fat to 7 per cent.
Clash of the codes
Professional cruiserweight bout over four three-minute rounds
Carlos Spencer
Age:
39
All Black tests:
35
Fight history:
4 amateur bouts, 3 wins, 1 draw
Job:
Coach of Eastern Province Kings
TAB odds:
$5 outsider
Monty Betham
Age: 36
Kiwis tests: 8
Fight history: 6 pro fights, 5 wins, 1 loss
Job: Manager, personal trainer at Boxing Alley/broadcaster
TAB odds: $1.15 favourite