"I was the official referee," Rob tells us ... "and good thing I was the official referee. My referring skills were pretty legit, 12 out of 10!"
The Gronks were trained up by two legendary fighters -- Winky Wright and Antonio Tarver ... and Logan says his main focus point was avoiding getting rocked!
"They're massive," Logan tells us ... "I'm f**king 6'2", 200 pounds and I go in there and all of Rob's brothers are towering over me! They're all taller than me vertically, they're all wider than me horizontally. These are huge humans! "
"Honestly, I walked in and I was like.