People will believe anything
As a modern species, we are notoriously bad at noticing things. Our lives tend not to depend on being able to tell the difference between this bush of tasty berries and this bush of poisonous ones, or which hillock the pride of lions is behind. Therefore, our powers of observation don’t get as much practice as they should.
This could be the explanation for why this manufacturer thought his customers wouldn’t notice if he relaunched his product in a smaller bag, but added the phrase “new bigger size” on the top. If the two packets aren’t side by side, it isn’t something you’d notice, but this amateur detective solved the advertising crime.