RS,
You're right - it doesn't make sense.
Larger lines will have no effect.
If you open the bleeder fitting, can you push the brake pedal go all the way to the floor?
Frankly, I think it's the front pads getting glazed. The rear only contributes about 30% of the braking. I realize that you haven't been replacing the front pads, but are you "breaking in" or testing the rears after new pads, possibly deglazing the fronts?