Whether players are fitter/stronger is a debate for another day, but reducing the depth of cricket bats will sort out the men from the boys. You see in modern cricket every Tom/Dick/Harry pounding the ball over the fence. Mishits that in years gone by would be simple catches are now disappearing 20-30 yards over the fence. That's not properly batsmanship.
Reducing the bat sizes wouldn't effect the elite players(AB, Kohli, Smith, Warner, Root etc) it will show up the lesser players like the Maxwell's/Miller's/Buttler's of the modern game.