bornriding - The Carrier spec shows the same wheel for both motors (10x6).
eisenberg - I am not a motor expert so I would wait to see replies from some others on this site that may know more about motors before changing anything just yet. The Carrier spec should be correct unless there is a typo in the documentation. I also would wait on changing duct sizes until you decide what you are going to do about the CFM issue.
How high are your ceiling registers and is the return high or low?
BTW: The motor only runs RPM, it is the wheel ( or cage ) that determines CFM, at whatever RPM the motor turns
CFM and RPM are proportional "Fan Laws"
Fan Law #1
An increase in CFM = a direct proportional increase in RPM
A decrease in CFM = a direct proportional decrease in RPM
Static pressure changes as the "Square" of RPM change/Ratio
and of course Brake horsepower or just horsepower changes as the "Cube" of the RPM or CFM.
So the bigger motor has more RPM and produces a greater CFM so it must have a greater HP rating as per fan laws.
P.S. Forget changing the motor and go here---->www.fanhandler.com and enjoy your air conditioning!!