That's an awfully short statement / quite a baseless comment. What does Costco even sell, are they a normal supermarket? Are other supermarkets not at fault? What does Costco do wrong compared to competitors (even if they are not alone, you seem to think them particularly responsible)?
Costco's thing is that they're bulk quantities at a lower price. They'll either have larger bottles/bags/cans, or they'll package 2+ of the regular ones together.
At my local supermarket, I can buy a single onion if I'd like. If I want a them pre-bagged, it's a 3LBs bag. At Costco, the option is a 10LB bag. 5LBs bags of mozzarella. 5LB and 10LBs bags of carrots.
It's a great place to shop, especially for large families/groups, but if you're a small household then it will lend towards over-buying, simply because you can't get a 1-2 person quantity of most things. This morning, I walked to the store and bought a single avocado. If I'd gone to Costco, I would've had to get a bag of five. (Which is surprisingly few for Costco.)
