Sitting on the landing gear absolutely removes weight from the axles vs the pin, but it isn't a lot of difference. If you know engineering it is just simple statics to compare them, solve the system of equations that sum the forces in the vertical direction and sum the moments about a select point.
Conceptually think of it like this. You know the center of gravity is forward of the axles because there is pin weight. So if you take the support at the pin and gradually move it back towards the axles, what happens? At some point the support is under the center of gravity and thus supporting and balancing the entire weight of the trailer, the axles are now not supporting anything and the trailer could tilt back and forth like a seesaw. So if you support it at the jacks, they will carry more load than if supported at the pin, so the axles support less. It won't be much different, on the order of a few hundred pounds, because the difference in distance from the center of gravity to the jacks vs the pin is small. The exact amount depends on the ratio of COG to pin divided by COG to Jacks
