The surface area that matters is the area above the ph solution. A funnel shape at the aquarium water side doesn't change that.
I was thinking that both would matter quite a bit. The way I was thinking is that you want CO2 from aquarium water to get to the ph solution. In order for this to happen it needs to get into the trapped air in the drop checker first, and a larger surface area would speed this up. The faster you can get CO2 into that trapped air, the faster CO2 can enter the ph solution, which you would also speed up by having a large surface area.
I do also see having the least amount of solution and air in there being beneficial. Seems like the lower volume of both, the faster it can reach an equal level of CO2 with the tank water.
I don't mean to sound argumentative about it, I'm just trying to understand the reasoning behind the ideas.