The output power of a motor is dependent on the internal construction of the motor (e.g. the number of windings in the coil, strength of the magnets, number of coil and commutator segments), so while the physical size could limit the capabilities to some extent, it does not give a definitive indication of the power output.
We currently only carry one version of our 37D motor, which is available with various gearboxes, but you can see an example of how motor specifications can vary in motors of the same size by comparing the low-power, medium-power, and high-power versions of our 12V 25D metal gearmotors.
By the way, just to clarify, the 131:1 metal gearmotor that you are asking about has a rated speed of 80 RPM at free-run (no load) and a rated stall torque of 18kg-cm. We generally recommend these motors be ran at about 25% of their stall torque. More information about the gearmotor (including a speed vs torque curve, the maximum efficiency point, and the maximum power point) can be calculated using the methods described in the “FAQs” tab of the gearmotor’s product page.