I don't know that much about this, but I've noticed the pedal stroke is longer - before it gets real firm - on my '92 EX than a '92 DX I drove a while back. Both were sedans but the EX has a bigger MC as well as booster. My car has new DOT3 in there, which firmed it up a lot, and I put on new Nissin pads not that long ago.
It's got good stopping power, as far as I can tell it's just the way the car is set up, which would support your theory of bigger diameter leading to a longer stroke.