Rad-C (you don't mind the moniker, do you, radfordc?) was talking about flow rate vs. fuel burn, not fuel burn vs. fuel capacity. In other words, how much fuel dribbles through the system in a given period of time compared to how much the engine burns in the same period of time. You could have a million gallons in the tank, but if you're flowing 5gph and burning 5gph you're on thin ice.
Good point. Of course, you don't have to be upside down to get extra head pressure (just a normal turn would do the same thing). Lots of planes with a gravity system and slide carb are OK with the temporary increase in head pressure. The thing to remember is that you're not pulling extra Gs for too long and the slide carb is tolerant of the temporary minor extra pressure. The needle is a flow restriction and I believe that just because you have 2x the head pressure for a moment or two, it doesn't mean that you get a mixture that's twice as rich. Maybe somebody could correct me if I'm mistaken.