Illusion and ThunderCactus both bring up good points. It may not be a bad idea to permanently post this info somewhere. A lot of countries actually use joules as their airsoft legal limitations. There is a direct connection between a BB's energy, and the gun it was propelled from. The basic formula being: velocity (speed) times Mass (weight) equals joules (energy). Other factors must be taken into account. Things like projectile aerodynamics, distance, gravity, and spin.
But illusion is correct. Joules rise when a heavier round is used. Even in a spring gun. In a GBB/GBBR you have to factor in gas expansion. For instance propane is compressed approximately 273 times, and has an expansion rate directly effected by temperature. Therefore the more time a projectile is plugging the barrel the more time it has to apply that force. Also as long as the gun hasn't finished it's cycle it may produce a larger amount of gas behind the BB. If the BB hasn't left the muzzle within this time then the gas will have more compressive force.