Doesn't Walmart own most of their real estate though? If so, they wouldn't be in the running for largest commercial lessee. Although it wouldn't surprise me if Walmart does have the largest square footage of commercial real estate, if you count parking lots & distribution centers it would be huge.

Another odd thought--would you count an oil company like halliburton's well leases? I know that square footage is normally used for buildings as to account for multiple stories, but if we count commercial land leases then either an oil/gas company or a large agribusiness would almost certainly come out on top.