As others have mentioned the rod reduces the piston area so the retract force is less.
Here is a handy chart from Bimba that give the power factor and the approximate force created with a 60 psi input.
http://www.bimba.com/global/library/...er_factors.pdf
There is also one on page 20 of this booklet
http://www.bimba.com/global/library/...efhandbook.pdf that was included in the KOP in the past. That page (and the entire booklet) includes some important design considerations that are well worth noting. For example there is a page on how to calculate the approximate extension speed of the cylinder and how to maximize it.