Seems to be the new trend with all food products as someone else pointed out. People are more likely to notice the increase in a few pennies, but not the loss of a few oz. Especially if they put it in a new container, since then any size difference is lost by the new look.
Personally I had not noticed a change in the ice cream, but then again it's a treat when I do get to have it so I don't notice price or the container size. I'll have to take a look now though, since what better questions to ask and answer for one self then those involving food and thier containers while shopping at the grocery store.