why the pellet speed decrease with a heavy nitro piston?

[Cenoura]

Hello everybody
I would like to know with the most experienced in tunnings springers, why when I use a heavy nitropiston the pellet speed decrease?
here in Brazil you can buy NP with 35-80 kilograms of force. I already tested a wide variety of settings and usually airguns in the best results are with 45-50 kilograms NPs (this is talking about the high-powered weapons like most Gamo, Bam, using NP 27cm long).  using a heavier, the  pellet velocity do not Increase, only increasing the recoil.
the popular hatsan 125 is best with a 70kg NP than a 75 or 80..
the gamo CFX, and others are best with a 45kg NP...
why its happens?

thanks