Gonna slightly disagree with Long Rat here...
Larger gears less wear (but you'll never notice!)
Larger gears mean more tooth engagement so more efficient transfer of power (but you'll never notice)
Larger gears will add rotating weight, but as a proportion of the total rotation weight of the rotor, axle, wheels and bearings... you'll never notice!
Shifting the motor weight forward makes little difference to the weight distribution as all the weight of the pod goes through the front ball joint of the pod. However, it will make a difference to the way the car turns. The further back the motor the more weight over the axle and more that weight will swing when the car turns.
Using a larger spur and moving the motor forward would make the rear swing less, giving you the effect you describe. This is the biggest effect of motor movement in pan cars - remember they are the only race chassis in the world with a hinge in the middle!!
If you are so close to the edge with you car's handling that moving a motor a few millimetres forwards changes the handling significantly, the as Long Rat says you need to look at your set-up compared to others in your club with the same car. HTH
