Floyd Mayweather ain't gonna like this ...
George Foreman tells TMZ Sports it doesn't matter if Money goes ANOTHER 50-0 ... he'll never consider him the greatest boxer of all-time -- 'cause he ain't a heavyweight!
"If Floyd Mayweather goes out and gets a real beer belly and comes back and challenges the heavyweights, I'll call him all that," George tells us.
"But right now, he's thin as a rail."
Floyd's 50-0 and owns victories over Oscar De La Hoya, Manny Pacquiao, Shane Mosley and just about every other great fighter of this era ... and even has a win against one of the UFC's best too (sorry, Conor).
But, Foreman clearly doesn't care ... telling us ya gotta beat the BIG boys to wear the GOAT crown.
"He's got to say, 'I'm one of the best.' That's all he can say."
Disagree with George? Perhaps ... but we ain't about to argue with him -- dude's still scary as hell!!