As you have seen I am researching the same cause. I am finding if you look at the torque curves of the motor running in series vs parallel the slope will be shallower for parallel.
So a smaller motor running at the same amperage in parallel than a larger motor in series will have higher torque at running speed.
To maintain a constant acceleration requires a constant torque through the range of rpm because of the inertia of the gantry.
Since stepper motors lose torque with speed the actual obtainable acceleration is not constant and decays as speed increases.
Now my issue is that Mach3 will only allow a constant acceleration value. So you have to pick you worst case torque value which will be one at higher rpm.
I am working on a calculator to look at this more closely in my other thread.