Ahhh...most Canon consumer photo printers do NOT have "different sized" nozzles for the same color. They are all the same size (Pigment Black nozzles are typically larger and fewer than color nozzles, but they are use for text/line graphics, only on plain paper). That said, some Canon inkjets might have different sized nozzles, but I'm not aware of any consumer (home) printers that do.The thing are not as simple as it sounds.
You have to knew that the most consumer Canon printers do have 2 - 3 nozzle rows for particular color channel. These rows are different in color intensity or in droplet size. Smaller a droplet the lighter color.
As you probably knew Epson has variable inkjet droplet technology: That means changing piezo impulse it can change inkjet droplet size
No, look to all CISS type assemblies, the air vent channel always goes to the bottom of the ink container, and this level is below the nozzle plate level. Cartridges with foam inside don't drop, the foam is holding back the ink, and Epson cartridges have a membrane valve inside preventing ink to leak when the printhead is in idle position, all such techniques create a very slight underpressure.most of ink is delivered by gravity