Force = Mass x Acceleration

Therefore, bat size and swing speed are equally important. If you DD can swing a 22oz bat at 60MPH, the force generated is 1320 (22 x 60). If she moves up to a 23oz bat she would need to have a swing speed of greater than 57.4MPH to generate increase force (1320/23), and if she moved down to a 21oz bat she would need a swing speed of 62.8MPH to generate the same force (1320/21).