Because the sensing area upon which the oil pressure is acting inside the gauge stays the same size regardless of the line size to the gauge.
Hydraulic force on an area is simple: force in psi X area in inches = total force in lbs. I am sure we all know this as brake hydraulics work this way too. Put in larger caliper pistons with the same leg/line force and braking force goes up because the area upon which the pressure is acting goes up. (BTW, I'm an engineer professionally and do this stuff just about every day...... but I'm not sure if THAT helps convince anyone at all LOL.)
Agreed on things being close as far as leakage....