You aren’t just seeing things.  Over the past few years, the majority of cake mix boxes have shrunk.  What used to always be 18.25 oz is now usually 15.25 oz.   We have accounted for this in this Cake Mix Cookie recipe by adding a little extra flour.  But, if you happen to purchase a larger (18.25 oz) box of cake mix, just use the entire box and omit the all-purpose flour.