It means his hands are so Big he won't need a clean hit for it to feel like a sledgehammer
He won't need to hit Haye a clean shot having an extra 7 stone behind it
Without getting technical this is how a KO works... your brain is floating in fluid (seribral fluid or something its called), then certain areas of the head receive trauma (chin, ears & the behind them, 4 places on the back of your head and the temples) your brain wobbles and impacts on the skull. Thats when the lights go out. If you spread power like that over a big area the KO becomes less effective. Boxers train for many years to take impacts like this as do most full contact fighters. Yes it will take your head off but its not an effective way of completing a KO. Besides the biggest KO areas on the back of the head are illegal in boxing anyways. Not to mention they are wearing gloves which spread the hit further. Big chance of breaking the nose and giving weird looking ears other then that less effective. Think of how say a bullet works compared to a bean bag less lethal equivalent the bullet penetrates the LL causes blunt force trauma.
The extra 7 stone isnt power either, look at the body and shape of the muscles Hayes has more raw power and it ripped to bits the giant is how can i say it nicely ... over weight.
Technically your are right tho a big fist has more chance of clipping the chin