You haven't given the specs of your present motors but I suspect that your *real* problem is the drivers.
According to the data sheet for the board it is 24v 1.5amp to 3amp.
Doesn't mean much without motor specs.
The motor supplier says
"Yes this driver will work with our steppers, you can use them in series to give you better torque but slower speed or in parallel lower power but higher speed."
Doesn't mean much - but it might get him a sale.
Why wont a bigger motor 566oz against 263oz not produce more power?
Beacuse (very simply put) motors don't *produce* power, they convert electrical *power* into mechanical *power*. If the electrical power isn't there in the first place, *more powerful* motors arn't going to improve anything.
Not a lot to go on but my guess would be you'd be better off replacing your drivers than your motors.
Ian