The food you buy really is shrinking

The box of Reese's Puffs looks different. You're not imagining it. Breakfast cereal is one of the many products that have been getting smaller.

In this article on BBC May 14, 2018:

A study of U.S. breakfast cereals over a three-year period by researchers at Arizona State University and Cornell University in New York found 15 products suffered a reduction in packet size, and in the majority this resulted in an increase in the relative cost for each ounce (28g) of cereal. 

— W. P. Carey Professor of Agribusiness Timothy Richards, who is the Marvin and June Morrison Chair in Agribusiness, is the ASU researcher who co-authored the study.