There are two angles.
The first is cost cutting. Pack sizes are routinely manipulated to hit certain price points, particularly for “single pricepoint retailers” – ie, pound shops.
The second is avoiding regulatory risk. The confectionery manufacturers have realised that the worst case scenario is that they could be the new tobacco industry. As a result, they’re gradually manipulating pack sizes down to avoid being made to put health warnings or GDAs on the pack. Which is why you can’t buy a “king size” of anything these days, but you can buy a two pack – you claim it’s a sharing pack, and instead of being 30% of a day’s calories, it’s 15% per portion…