Toilet Paper

A trend has developed for items to become smaller but cost the same. I wonder what is the psychology behind this.

For example , a 24 pack of No Name brand toilet paper has been shrinking steadily for about a year now. It used to be around 250 sheets per roll. When I last bought a package less than a month ago, it had 176 sheets per roll. Now it has 140 sheets per roll. That's a total of 3360 sheets for $4.97. The President's Choice "double roll" 12 pack has 352 sheets per roll, for a total of 4224 sheets at $5.76. On a cost per roll basis, the No Name is cheaper, but as a cost per sheet, it is even. Since toilet paper is used by the sheet, it would seem that the PC 12 pack is actually going to last a little longer.

I must be missing something here and thinking about it is giving me a headache.