Re: Washer bottle capacity (Rant alert).
There are two pumps in the washer tank, one for the headlamps and a smaller one for the windscreen. Yes - driving with the lights on activates the second washer pump for the headlamps.
The problem is that the volume of water applied to the headlamps is much greater than the volume applied to the windows. Just compare the size of the openings. When I use the washers, the spray bounces off the headlamps all over the car, some of it ends up on the windscreen which assists I suppose. However, in reality the sprays onto the headlamps are not that effective at cleaning the lenses. I still find myself having to wipe the lights with a cloth every few days even though the water is being applied.
I've always been meaning to disconnect the headlamp washer pump in the winter. This would reduce the time between refills by a huge amount in my opinion.
Might try it over the next few days, as driving to and from work in the dark on gritted roads, is needing regular refills.