ThunderCactus is spot on with the difference between large cells and small cells.
Small cells 2/3A cannot provide the same amount of continuous power to a system like a Large Cell SubC batter pack.
If you take the largest small cell (i.e. 1600 mAh) NiMH and the smallest large cell (1100mAh I think) NiMH there will still be a faster ROF and longevity difference with the large cell pack. Comes down to chemistry and the amount of goo that can react at once in the end.
Here's how you calculate battery power:
Large SubC pack made X brand cells, Y Volts, Z mAh + M[AAA] spring + [BBB] motor = Good to go all day
X can be:
Sanyo, GP, Intellect
Y can be:
Z can be:
AAA can be:
90, 100, 110, 120
BBB can be:
Guarder Infinite Torque Up
Systema Magnum (Thundercactus will say No)
*** careful mixing extremes...i.e. a 9.6v 3600mAh Sanyo pack with a M90 spring and a Infinite Torque Up motor might = stripped piston