Which isn't evidence of the aforementioned point. I'll grant you "exponential" isn't mathematically correct - it's more a turn of phrase. But the profit margins here are strategic.

Costco's gross income per square foot is much higher than Walmart's while their margin is half that of Walmart's. Costco is more valuable, and makes less profit. This is because Costco's aim is to provide the best value for its members [whom usually have more disposable income]. It does this by jockeying for the best bulk price, increasing efficiency, and providing good pay and benefits to its workers. Costco is a shining example of the idea of the corporation being a partner with the employees.

Walmart's aim is to control markets, and does so by eliminating the potential for price competition and then using its buying power to force sellers to bow to its whims. Costco could make much more profit by treating its workers the way Walmart does, but chooses not to, because its value proposition is different.

Costco will also probably not be able to compete for long, not because it isn't valued well, but because Amazon and Walmart will eventually take over every remaining obstacle to the consumer's wallet. Walmart is also slowly increasing its revenue per square foot so it doesn't have to keep fighting a losing battle online.