Once the bike is running, the alternator will basically run the lights and blinkers.
This depends on the RPM and the electrical load. The 550 Alternator is capable of producing 150 Watts @ 5000 RPM. It produces about 1/3 of that at idle RPM.
The stock bike uses about 120 Watts (lighting on), not counting battery recharge load.
So at idle, the bike absorbs more power than the alternator can produce and the difference is taken from the battery in depletion mode. The scheme works only because more operational time is spent above idle than at idle, and the battery has sufficient reserve storage to keep power available to the ignition system and total electrical usage. Reduce the battery capacity and not the electrical load, and you reduce the available percentage of time spent idling. In other words, if you keep the RPM above, say, 3000 all the time, the battery will likely keep it's full charge state. If you spend more time idling than riding, the bike will become very quiet and a subject of pushing, much sooner with a smaller battery.
Also, understand that the alternator design was created in the late 1960's, when separate lighting switches were routine equipment. It was assumed the lights weren't used during daylight hours. This reduced idle battery drain considerably to nearly match the idling alternator output. Later legislation removed the lighting switch and the alternator wasn't updated along with the mandated change.
