That helped one bettor win $166,666.70, but it required a bet of $1 million to get there. On Saturday, that didn't scare off the unnamed bettor from putting it all on the reigning featherweight champion.
After taking the wager, William Hill moved Nunes to -650 and Spencer to +450.
Nunes became the first UFC fighter to defend titles in multiple weight classes while simultaneously holding both belts. Spencer, meanwhile, will need to rebound after dropping her second fight in her last three matchups.
Before defeating Zarah Fairn Dos Santos in February, Spencer suffered a loss via unanimous decision to Cris Cyborg.
Nunes has now won 11 straight fights as she continues to prove herself as the most feared female fighter in MMA.