They are comparable at the level of "retail" and maybe even "big box retail", but that is about it. The 10 Walmarts within the same distance of me as the 1 CostCo sort of obviously aren't competing with CostCo.

Still totally reasonable to look at what structural changes could be made to shift costs towards Walmart (away from taxpayers), but the comparison to CostCo isn't really instructive.

And it could just be that Walmart shouldn't be able to run so many stores. But that's going to change the availability of good for many people.