Studies have shown that the Saturday newspaper is the least read of the week. Why? First, Saturday is a pretty busy day for most people and they don't have a lot of time to sit around and read. Second, most people aren't going to work on Saturday so there's virtually no newsstand sales, nobody picking up a paper for the train ride to the office, nobody reading the paper over their lunch break, etc.
Because readership is down on Saturdays, so are ad sales. The size of the newspaper (i.e. number of pages) is determined more by ads sold than by the amount of news fit to be print. Without ads, newspapers decrease the number of pages, hence a smaller product on Saturdays.