Walmarts can be 3-10x the size of Whole Foods - that's not quite fair...

I daresay there are aspects of both income distribution and other factors that favor or disfavor big box stores generally. No, a Walmarts footprint probably doesn't work for San Francisco proper but that doesn't really explain the relative lack of Walmarts in the Silicon Valley sprawl compared to the East Bay.

...and then you go to Dallas where you typically have a Walmart in walking distance of a Whole Foods or Central Market.