I've noticed this. The shops put the larger bags of crisps in the prominent position and the standard size on the bottom shelf hoping that people will just pick the bigger bag and spend more. Also, they are selling bags of Lays for 22 baht for 42 grams where they used to be 50g. If you buy two of the smaller 10 baht bags, you will get 50g for 20 baht. I've seen this in different shops. Not sure how long this will last as they will inevitably shrink the cheaper bags too.