The original and mosfet both consume all the unneeded current from the stator, basically it's a regulated short circuit to keep the voltage steady. All the extra current being produced by the stator that isn't being consumed by stuff on the bike is consumed by the regulator which creates heat. The mosfet does a better job and might run cooler.
A series regulator creates a controlled open circuit instead of short circuit. It 'disconnects' the stator so that there is no extra power being created.
Imagine that you had no throttle control so the engine is always running at full power. The only way to keep the bike at cruise speed is to hold the brakes, the excess energy from the engine is being consumed by the brakes, that's a shunt type regulator. The series regulator is a throttle. You added a higher output stator so any time that your accessories aren't on there's more power being consumed by the regulator. In the stator/engine analogy you installed a more powerful engine pushing against the brakes. Turning on your extra accessories would be like putting on luggage, it'll eat up some engine power so you can ease up on the brakes.