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 .