It's called 'shrinkflation' and is widespread. Rather than increase the price they reduce the contents. One example is Kellogg's cornflakes, where 500g has been reduced to 450g. Another example is that if you smoke then most of the cheaper brands will only have 17 cigarettes in a '20' packet.
Oddly, the personalised boxes (which seem to be temporarily sold out) still come in 12s (unless, of course, the new stock will then be in 10s):
http://www.mcvities.co.uk/products/jaffa-cakes
Of course, while McVities invented Jaffa Cakes, they didn't register the name as a trade mark, so there are plenty of other brands available (especially supermarket own-label ones), which are often just as good (and almost invariably cheaper)
The box itself appears to be the same size as before. Stand an unopened bag of Jaffa Cakes against the box and the box is much taller than the stack of Jaffa Cakes. It looks like they removed the two Jaffa Cakes but kept the bag and box the same sizes.
Late decimalisation ! Conned us in 1971 too. One doesn't have to lie in order to con. Just create a difference between expectation and reality. Then it's a legal con.
To be fair to manufacturers, they're often forced to reduce the weight/size/quantity of their products because major retailers insist upon holding to a certain price, even when the manufacturer is facing increasing costs for things like ingredients and staff pay.
For example, the reason that Toblerone bars kept getting smaller and smaller was because the main sales outlet for them in the UK was (and possibly still is) Poundland. The retailer demanded a product which could always be sold for a quid, forcing the manufacturer to reduce the size of their bars.
Similarly people often complain about the ever-reducing weight in the tubs of Roses which are widely sold at Christmas. The reason it happens is because all of the big supermarkets demand a product which can still be sold for the same £5 price that they were charging over a decade ago.