Nestled near the Columbia River in Wood Village, Oregon, is the largest Walmart in the Portland region. The building spans three-and-a-half football fields, but it’s dwarfed by something else: the surrounding parking lot, twice as big as the store itself.

When it expanded from a Walmart to a Walmart Supercenter in 2004, its floor space increased by 45 percent. The parking lot grew less, though, only adding 36 percent more spaces.

That’s because, as it turns out, Walmart has been quietly reducing its parking ratios for years.