First that's a comparison for the shopper, not the retailer (and we are discussing the retailer here).

And second, you can only buy at Costco if you need the quantities they sell. Everything you can get at Costco you can also get at WalMart, but the reverse is not the case - it's not "a few extra thing" - Costco doesn't carry a huge amount of stuff you can get at WalMart.

So for the consumer buying at Costco is exclusively about what quantity they need. (Or if they have a lot of storage space + budget to store extra.) And then they also have to go to WalMart to fill in the rest.

Or in other words they complement each other - they are only partially competitors.