As we previously reported, Logan invited Rob and his 4 bros to a training session as he gears up for his big June 6 match against Floyd Mayweather.

The bros -- Glenn, Dan, Chris and Gordie -- all agreed to take go 1 round each with Logan ... full speed, fun strength with Rob serving as referee.

First up was Glenn ... a 6'3", 234-pound former NFL player who spent time with the Patriots.

Look, Glenn put up a good fight but it was clear he was just outmatched by Logan who dominated until they stopped the fight.

Round 2 was Chris Gronkowski's turn ... the former NFL fullback got rocked right away and the fight was quickly stopped.

Round 3 featured Dan Gronkowski -- a former NFL tight end who played for the Lions, Broncos, Pats and Browns.

Dan did not do well -- getting rocked hard and dropped. They stopped that pretty quickly, too.

Finally, it was Gordie's turn ... and Logan smothered him with a barrage of power shots until Rob stepped in and pulled them apart.

After the session, Rob praised Logan for being a beast -- and gave respect to his bros for having the balls to take on a guy who's been training like a maniac for the past several years.

Bottom line ... Logan Paul's no joke but Floyd Mayweather is on a whole other level!