COSTCO is 1/5th the size of Amazon, even though the company has been around about 20 years longer.
That's still a _lot_ of freakin' restaurants my guy. Costco is the only counterexample that I know of to "all workplaces need unions in order to guarantee workers' quality of life", but I think they might be the exception that proves the rule. That they've grown to be that big and treated their employees that well is a testament to how good economically, long term, it is to treat your employees as the valuable assets they are.
I also trust Costco a lot more than Amazon, even though Costco is not as convenient to use due to their smaller number of SKUs.
The idea is that if you see something that interests you, you buy it now, and there's always something new on each visit. It's annoying for certain things, but at the same time it allows them to avoid having the same stock on the floor because people are on the fence about a purchase.
So pretty freaking big?
Personally I spend most of my money at Costco when I can because I prefer giving money to companies that treat their workers better (or at least better than Amazon).