Big packet, lots of crisps! Except when I opened it:
The crisps took up less than 50% of the volume of the bag. The rest of it was empty. I'd bought a bag of air with a few crisps at the bottom.
I dare say there's some formal explanation for this practice of padding with air, based on the not entirely false supposition that the packet acts as some giant piece of bubble wrap to stop the crisps inside from getting broken. However, the real reason for it is, of course, that people think they're getting more crisps for their money than they really are.
I knew I was getting 150g of crisps, because that's what it said on the packet. However, I didn't know the density of kettle chips, so I didn't know how many that would turn out to be (they seem to weigh much more individually than regular crisps).
Maybe in future I'll just assume that all packets of crisps are only half full, and make my buying decision based on that.