Drives me nuts. The reduction in size for me often means there's no longer enough to cover 4 people and I have to buy two of something I previously needed only one of. This doubling in cost annoys me so much the brand ends up on my shit list and I'll go out of my way to stop buying it.
They think people don't notice but most of us do. It's instinctive to look at how much food (in volume) there is. I'd rather a small increase in price rather than doubling of my bill.