There are several factors that determine the output of a pickup, here are a few:

The coil shape and number of turns of a particular wire gauge.
The magnetic material and shape used to magnetize the strings.
The magnetic path of the magnetic field to the strings.
The type material used to conduct the magnetic field to the strings.
The position of the pickup under the strings-near the bridge or neck.
The pickup height to the strings.
The string gauge and tension.
The body and bridge material.
The controls, wiring and cable quality to the amplifier.