
There are two main ways to grow a catalog business: one is to increase circulation; the other is to increase page count.
If your catalog weighs less than 3.3 ounces, increasing page count won't increase your postage cost. If your catalog weights more than 3.3 ounces, the incremental increase in postage will be marginal if you add additional pages.
Page count does make a difference when it comes to maximizing response rate. Any increase in page count (up to a point) will increase response, and therefore sales. In fact, sales will increase by approximately one-half the percent increase in page count. For example, a 20 percent increase in pages will increase sales by approximately 10 percent. Of course, this is a rule of thumb.
There's a favorable relationship between the incremental costs of adding pages vs. the actual return. Pages generate a high return on investment. For example, increasing page count from 52 pages to 60 pages yields a 15.4 percent increase in the number of square inches of selling space. Yet, typically the cost is only 7.4 percent more for the eight extra pages (again, this is approximate depending on the quantity printed).
Adding pages means maintaining the proper page density. It doesn't mean that you should devote more space to the items being added. Nor does it mean that you should give more space to existing products simply as a way to fill more pages. For the economics to work, proper density must be maintained as page count is increased. If you typically put eight items on a page, maintain that same product density. Simply put, the bigger the store (i.e., the catalog), the greater the sales. Often when pages are added there's a tendency to want to promote "the brand" vs. selling merchandise. Keep in mind that every page in the catalog except for the front cover should generate measureable sales and profits. Think of a catalog as a piece of real estate, where every square inch of selling space generates a direct return.

Steve Lett graduated from Indiana University in 1970 and immediately began his 50-year career in Direct Marketing; mainly catalogs.
Steve spent the first 25 years of his career in executive level positions at both consumer and business-to-business companies. The next 25 years have been with Lett Direct, Inc., the company Steve founded in early 1995. Lett Direct, Inc., is a catalog and internet consulting firm specializing in circulation planning, plan execution, analysis and digital marketing (Google Premier Partner).
Steve has served on the Ethics Committee of the Direct Marketing Association (DMA) and on a number of company boards, both public and private. He served on the Board of the ACMA.  He has been the subject of two Harvard Business School case studies. He is the author of a book, Strategic Catalog Marketing. Steve is a past Chairman of both the Catalog Council and Business Mail Council of the DMA. He spent a few years teaching Direct Marketing at Indiana University in Bloomington, Indiana.
You can contact Steve at stevelett@lettdirect.com.