There's often quite a difference depending on which container size you buy, and it's not always the case that the largest container is the best value.
But would I notice that last week I was getting a 20oz package and this week it is a 19oz package? Not sure. I do remember this happening with candy bars as a kid. The price would be the same, but the bars would get smaller and smaller. Then suddenly the price would go up, the bar would be bigger, and the cycle would repeat.