Most people seem to be completely confused about the difference between the PGI and the CLI cartridges in a Canon inkjet printer.
I will try to explain the differences and why they exist. Firstly the Large Black "PGI" cartridge in most Canon printers stands for "Pigments". This is the cartridge which gets used when printing Text. Pigment based inks are "more blacker" and provide a crisp, dark printout which makes them ideal for text. Also pigment particles tend to be larger and stand up more on the paper which results in almost a "Laser Printer Quality" text printout.
The other smaller cartridges are known as the "CLI" tanks. This stands for "Chromalife" which is a dye based ink formulation. The ink in these cartridges is specifically designed for photo printing. Dye based inks tend to have smaller particles than the pigment based inks and they tend to lay flatter on the paper. This results in more bright, vivid printouts.
Most Canon printers use either 3, 4 or some even 5 CLI cartridges. Many newer Canon printers such as the Canon MP980 are also incorporating a Grey CLI cartridge to improve on the various grey scales.
People often ask the question as to why their colour inks are getting used up when they only print in black and white. The Simple answer is that every time your printer goes into cleaning mode (thats the weird dance the printer does when you are in a hurry to print) it pushes ALL of the inks out of the print head and onto the waste pad. This is the method which all inkjet printers use in order to stop them from clogging up.
So to sum things up if you set your printer to Photo Paper it will use the smaller CLI inks and if you have it on Plain Paper setting then it will also use the PGI cartridge especially when printing text. Happy Printing ! For more info take a look at the following video clip about various inks.