To be fair, you do partially have a point there; the official Canon printer drivers certainly support more of their own printers than CUPS does. I can tell you that without even looking at Canon's official driver install. However, the total amount of printers supported by CUPS, since it includes a sampling of most major manufacturer's printers (and all of the features of most of said printers) utterly dwarves what any one manufacturer supports currently in their own drivers in Windows. Yes, the average age of the list doesn't necessarily include as many printers released THIS YEAR (another partial point to your statement) but it also doesn't exclude printers that used to work simply because they're old enough that Canon wants you to buy a new one so they simply merged out support. What you're getting in that 17MB (probably less than 10MB really, for normal users - my installation case is an exception because I use multi-arch and compile a lot of packages on my own) is basic or complete support for a broad cross section of printers going back for more than a decade, not just the most recent offerings of one manufacturer's last 2 years of flagship products. Note this figure also includes the documentation.
But that doesn't really mean Canon's software is in and of itself bloated and horrible necessarily. If I had to bet on it, my guess would be that 28MB of the 30MB used by the Canon driver install is a hidden video of the developers eating birthday cake.