It's a bit cheeky indeed, but not too uncommon for brands to make the product look and cost the same, yet either changing the amount of products in the package, or even change the amount of ingredients in a product. The latter is the worst i.m.o. but it both sucks when you don't expect it.. Of course i can imagine why after such alterations, most of the brands don't feel like including the message "now with 25% less!" or "now only 10% cacao-butter, but 100% more sugar!" etc., however i do feel that they should do just that.
I am not quite familiar with the laws of false advertising, and/or changing of amounts and ingredients within an edible product, but i am sure there are plenty loopholes for the brands, or this wouldn't happen. And it happens a lot.
My favorite chocolates/toffees from England are Quality Street, and over the years they kept the package pretty much the same size, but have definitely decreased the amounts of candy in there.. bit of a bummer, but as long as they don't mess with the recipe i'll still buy them, that is, until i find only one little fella in the box
Anyway, sorry "for your loss"