This is an increasing trend - the price stays the same but the quantity you get for the price is reduced. This can be a decrease of more than 10% in some cases. Products which have done this range from your favourite bar of chocolate to your washing up liquid via tinned soup, loaves of bread and countless other products. In some cases the quantity is going down and the price going up at the same time!
Now, I have no major problem with increasing prices (well, we'd all prefer prices went down but that's not going to happen) - as long as they're justified. But why the subterfuge? Why not just keep the quantities the same as what everyone is used to and put the price up? It would seem the honest way of doing things. I know myself that I will tend to avoid products where this tactic has been used - I hope everyone else does the same!!