[BrianG]

Oh no, here we go again! Griffin and I had this discussion a while back.

The vast majority of a motor's "resistance" comes from the back-EMF. A slower turning motor under heavy mechanical load reduces the BEMF, thus lowering the "resistance" and increasing current flow. By reducing the gearing, you are allowing the motor to spin faster doing less work for each rev. Since the motor is spinning faster with less load, more BEMF is generated, thus increasing resistance and lowering the current.