You sort of answered your own question there.
Spring motors store mechanical energy that gets put into them, and then release it into a system, usually with some sort of governor that exchanges the release of a small amount of energy (usually to spin something up) to get a more controlled release.
So, a longer/stiffer spring that takes more energy to wind, will release more energy when unwound, There's always loss in any mechanical system due to things like friction and wear, but a spring motor is just like a battery, the more energy you can stick into it without it exploding/degrading, the more energy you can get out, as energy can't just disappear.