Unless the weight savings is worth it.. (i doubt it) I believe that the only reason for a gear reduction starter would be to "mask" another problem like a poor electrical system, or increased friction in the rotating assembly like worn bearings, a marginal oil pump or a dragging clutch.
I definitely see the value in a gear reduction if you have purposely increased the compression of your motor... other wise i would look for other problems.
A DC motor can spin to 20,000 RPM. Hence, gear-reduction will provide more torque to spin the engine and use a smaller motor that is spinning faster. (Spinning faster = smaller rare earth magnetes = lower weight).
This is commonly used on high-compression, high-displacement V8's where you need all that torque to crank the motor over.
A good idea nevertheless. Benefits in smaller, low-compression (turbo) motors would not be so great....
Both lighter and shorter than the OE starter motor.
Be aware though that these high torque starters have a problem with the drain holes facing upwards when used on the Renault gearboxes (not sure if they fit the Citroen gearboxes). It is a relatively simple job to disassemble the motor and drill new drains on the underside and silicone the old holes up. There is an article on LEW about removing, disassembling and refitting one of these starter motors due to water ingress that resulted in the +ve being shorted out to ground.