Mike is reading four blogs. This is none of them.
An operating system (OS) is much more than a usual computer program. It provides an uniform layer (interface) to different hardware platforms and thereby helps other programs to run on different computer systems without having to adapt them. Of course an OS is much more. It takes care of those parts like disk drives, memory, graphics cards, input devices and all other components representing a computer system. It can manage multiple users at the same time, sharing resources like printers, protect from attacks and offers a user interface.
Most application programmers don't care much of size. All what matters is a broad range of features. The user want it. The user get it. Most application tend toward these direction and become fatter and fatter over the years. One day they have incorporated so many features that the underlying OS seems so paltry that it could be replaced by the application itself.
I see the future of the blog as a web operating system. - Matt Mullenweg
The same is true for databases, computer games, routers, browser and even word processing. Maybe some day you get your mobile with a pre-installed facebook OS - the very first Social Operating System.