When I worked for a high st catalogue retailler we trialed the mail order catalogue in a smaller size. Unsurprisingly these were very popular as they were around a third of the weight of the normal catalogue.
Most people were quite surprised that the text was still readable, due to improvements in printing technology. Even so, we bundled a magnifying sheet with the catalogues.
My guess would be that if the outline size reduced the thickness would increase unless, as anchor said, the print was smaller. Given that the whole process would need revamping for a different page size, an additional cost would be incurred which would be passed on to the consumer at some point.