Last I knew the difference in tank size was the plastic tank was the large tank, and the steel tank was the smaller tank.

Im not seeing where the length of the filler tube into the tank would regulate the tank level, the top of the filler neck is still way above tank height, so the fuel would still fill, and any air would vent out the vent.

It may be the pickup has a hole in it, or my other thought was a shorter pickup tube had been installed at some point.