I worked on one of those a few months back, if it's the same inside as the one I worked on it has a rather unusual mechanism for regulating the shot dosage. If you open it up and see a clear plastic water tank in the back left corner with a large number of level probes inside it, then it's the same. The way it seems to work is that it fills the tank up to the level of the highest probe. Then the rotary switch on the front selects the dose you want, which selects the appropriate probe (presumably the bigger the shot, the lower the probe sits). Hit the shot button and the control box activates the pump and the group solenoid until the control box senses the water level has dropped below the relevant probe.
Adjusting the shot volumes would presumably involve a bit of trial and error to determine which probe corresponds to which position of the switch (should be able to check that by carefully running the machine with the panels off and watching which probe seems to trigger the shot to end at each switch position) then adjust the volumes by adjusting the heights of the probes.