unfortunately piston fillers are filled by sucking up the ink directly from the bottle using your pen and the ink fills the barrel of the pen, so refills are not something that equates to piston fillers.
I do agree that refill cartridges, those little plastic tubes, or it's "converter" equivalent hold pretty much next to nothing in terms of ink capacity. I could run out of ink in a week or so, hence I got several fountain pens on the go.
by the way converters are a hybrid between piston fillers and cartridges. It sucks up ink from the bottle from dipping your pen in there, but it looks and attaches onto your pen much like a cartridge. Converters and cartridges can only be used in pens that utilize this method of filling.