I used to buy a certain type of margarine. Same container for years and years, for roughly the same price. One day, I opened a tub and thought it looked like it contained less. Sure enough upon inspecting the label, I found that it was now 15 oz. instead of 16 oz. Now, I look more closely to make sure I'm getting the same amount or at the very least, not paying more for less.
I always make it a point to check on the labels not really for the amount but for the nutrition content per serving. I am very conscious of my caloric intake so for me it's really important that I am always aware of the nutrition and complex molecules I will be putting in my body. I generally avoid those that are high in fat, and though I already have the idea which are which, I still double check. I think that everyone really should check on things like those.
As I'm vegetarian I always check the labels and quantities. Often I find all kinds of ingredients I did not expect like chicken broth i vegetable soup. Recently my favorite chocolate bars seem smaller and they have cut the weight, but kept the same price. Do they think no one notices? It's a ploy, they cut it so it's barely noticeable and then say it's to keep prices down, or their profits up.
That is a really good point, Theo, about finding unexpected ingredients. It's really important for people who have allergies and such as well. I do read labels for lots of information, but I specifically look at size as well now. I've also found that sometimes the "economy size" of a product costs more per unit than the smaller option does. That's another trick that can get by us easily if we're not paying close attention.