BAck in the early-mid 80's, I worked in a grocery store while going to college. Decreasing the amount of a food product while keeping the price relatively flat was really starting to be more widely employed at that time. Most people never noticed it. I remember canned veggies that were 12 oz. would start being 11.5 oz. I believe that it eventually got down to 11 oz. in a couple of years.
This is nothing new at all. The supplier knows that a small decrease in amount while not raising price is much less noticeable and will be less likely to have an impact on future sales.