I don't know the bat weight used by Bairstow or Jason Roy. I guess it is 2-10 to 2-11. BUT, and this is a big but, both Roy and Bairstow are fitter, stronger, and bigger than an average clubbie. Proportionally, and by that I mean strength wise, Roy and Bairstow are also using lighter bats. Same for J Butler. Let me clarify, for their fitness and strength level, a 2-10/2-11 bat is light hence the whippy bat speed.
I look at Bairstow's shots and I wonder how he is generating all that power! His shot is a short arm slap and no follow thru as in NO full-blooded swing. Things that make you go 'hmmmm'.