Assuming the hydraulic actuators (Jacks) are double acting cylinders, fluid is displaced from the return side of the actuators by the internal pistons as the actuators perform their stroke.
The hydraulic fluid displacements are not equivalent between the up and down stroke of the actuators since the actuation shaft displaces fluid from one side of the piston and reduces available piston surface. Normally you would require more force (thus more hydraulic fluid) to retract the landing gear than to extend it, so the non-shaft side of the piston is generally the one pressurized for retraction.
This applies to hydraulic systems in general-yours may be different. I am not at all familiar with the details of the 737 landing gear.