If it doesn't look different, customers might not know it's a different product than before due to the quantity change, and won't be prompted to investigate further.
Indeed, if the packaging is the same except for the quantity labels, then you don't, in fact, know it's smaller, because both the quantity change and the packaging change has been calculated to hide that fact from you. That's the problem being solved here.
With a proper packaging change, on the other hand, your confusion is resolved within seconds, when you read the packaging, which can include whatever explanatory info the manufacturer decides to put there. And if it's not resolved? The manufacturer should do better to explain.
