A pickup doesn't have more gain, per se, but certain pickups hit an amp's preamp stage harder than others. This drives youR amp harder and, in turn, usually gives you more distortion.
Now, to answer your question: What determines output?
The answer is that there are a lot of things. To give you a general guideline though, remember this:
Between two pickups, both with the same magnet type, the pickup with a higher resitance will hit your amp harder. Between two pickups with the same resistance, the one with the stronger magnet will hit harder.
The confusion starts when you have pickups with differing mags and resistance figures. At that point, you just have to try 'em.