Here's a cool map put together by Broadview Networks using data from Hoover's to show the biggest company, ranked by revenue, from each US state:
One big thing you see here is that defining "largest" in terms of revenue makes a significant difference. The most valuable company in Washington state is Microsoft, because selling software at massive scale is insanely profitable. And Amazon has a market value that's more than triple Costco's, because investors (apparently) believe that someday Amazon will abandon its zero-profit business strategy and emerge as some kind of monopolistic titan. But Costco's actual sales exceed either of those high-tech companies.
You see the same thing again in California, which is represented by a big oil company rather than by Apple or Google. Chevron is less lucrative than those Silicon Valley giants because the costs involved in its industry are enormous. But Chevron sells more dollars' worth of stuff.