Originally posted by QuackingPlums
Ok, batteries are made up of power cells.
The output voltage is determined by the number of cells. To cram enough cells into a 9v battery shell you need to make the plates really thin, which in turn increases the internal resistance, which lowers the output current. The sheer number of cells in a 9v battery also reduces the amount of electrolyte which equates to a reduced capacity.
For a non-force-fed loader there is little resistance to the paddles, so the motor doesn't draw as much current. A 9v would discharge much quicker in a Halo than in a Revvy or Egg, and you'll probably notice that the ball stack isn't under as much tension as the motor will struggle to wind up the spring.