A mechanical device to stabilize the stack height position of a compiler output tray and compress the sets in the tray is provided. The small mechanical device may be mounted in the back wall of the stacker tray near the top limit. The mechanical device includes a gravity-activated arm that moves out of the way when a set is ejected and, then, when the stacker tray lowers, the arm returns to the extended position. The tray rises again to the compiling position for the next set, pushing the arm upwards and compressing the set against the rubber contact member. The compression reduces the stack height, increases tray capacity and prevents each new set from pushing out the top of the stack.