I have a 60w/55w H4 and I'm about to change it to a 35w/35w (hopefully) because of this. As I have detailed elsewhere, i have had a persistent problem getting the bike to idle after it has been running for a while. My bike ('79) was sold with a 40w/30w headlight and the charging system seems not up to the task of something much larger.
For a long time, I never had a problem, but then I was hooking it up to a smart charger after every ride until the next one. When the weather got warm, I stopped hooking it up every time and after a few rides without hooking it up, my bike began to die slowly at idle. I tried everything- timing, carbs, etc.. On Saturday, I hooked the bike up to the charger and when I rode Sunday, it ran an idled perfectly. The only difference was the fact that I charged the battery. I'm hoping that this is the problem. I have a 35w/35w headlight bulb that I bought a while ago because others had reported problems with the 60w/55w. After I installed the big watt bulb with the Candlepower H4 conversion, I did not have problems, so I left it in and left the 35w/35w on the bench. I am beginning to think it was my habit of charging after every ride that prevented problems. I'm going to change to the 35w/35w and see what happens.
Of course, I will probably check it with my multimeter before changing to see what it is reading for voltage at idle, but I'm guessing that after a decent length ride, it's not enough. These bikes require a charged battery for ignition. The ignition will not work without a good battery even once running. If you have a smaller battery, you have less excess capacity and with the bigger watt headlight, that may just be too much.