I am using Honda CD 100 ss 100 cc two wheeler. If I increase the head light bulb watts little bit, will the mileage per liter of fuel reduce? and if so, any relationship?
Not by any amount you can measure, no. The power consumed by the bulbs are such a small fraction of the total that you would not notice it, unless you are adding multiple spotlights or going all retro-mod, maybe, like this pic from the Telegraph: