OK look at it this way...

If you use a larger MC, but change nothing else, the ratio of the MC area to the caliper piston area will increase. If it's 1:70 and you double the size (area) of the MC, it becomes 1:35. So to produce equivalent stopping force, you have to push twice as hard.

Take this example to its limits--with equal area MC area and caliper piston area--and you'd have to push pretty damn hard to generate that 1800lbs of force!