Higher fluid velocity diesn't mean more force. I'm more inclined to think that since the front calipers are 2 or 4 piston they will require more flow than the rear single piston calipers hence the larger diameter pipe. More piston surface area means more flow required.
That being said a lot of MCs have the front circuit closest to the fire wall. There is often a spring and proportioning valve between the pistons in the MC. The front circuit is acuated first and then a spring transmits the force to the next piston in line that feeds the rear. If that were the case then your MC has the fat pipe to the rear circuit.
The rear circuit is also further away and the longer a pipe is the less it will flow. Something to do with drag against the internal walls of the pipe. So a larger pipe feed int the rear would compensate for the reduced flow of the longer line.
They may have also used different sizes s the nuff nuff tech in the service department doesn't geth them mixed up
Cheers