Canon PGI-225 is the large black cartridge and the Canon CLI-226 is a small black, but the page yield of Canon PGI-225 is 340 pages while the page yield of Canon CLI-226 is 510 pages.
The two black cartridges are made from different material.
The large black (PGI-225) is a pigment ink, solid particles suspended in a carrier liquid which evaporates. This leaves the color particles on the surface of the paper where they bond to the paper by grabbing on to the almost microscopic rough surfaces of the paper fibers. This cartridge will be used when printing text documents or when printing in grayscale.
The small cartridges, black and colors (CLI-226), are all dye based inks. They are made to soak into the paper and dry. Since the black pigment which dries on the surface can't mix with the color dyes which soak into the paper they need to use a black dye to mix with the colors for a richer color palette. Despite the fact that it's black, the small cartridge is considered a color cartridge because of this.
Although both of them are black, they are used for different purposes. Pigment-based black PGI-225 is mainly used for text printing while Dye-based black CLI-226 is mainly used for picture printing. Therefore, the page yield doesn't depend on the ink volume only, there are many different variables that can affect page yield, particularly the content and type of document printed.
We provide the page yield information on our product page, which was tested by ISO/IEC 19752, 24711, and 19798. Regarding these test standards, please read the article: “What is Page Yield?” for details.