is the pentagon the world's largest building?

For whatever reason I had believed this. My understanding was that it had taken the title from Chicago's Merchandise Mart in terms of total square feet of space.

The Pentagon has a floor area of 610,000 square meters (m2), whereas the Merchandise Mart has 390,000 m2. So in that sense, the Pentagon is bigger than the Mart. However, there is a building in the Netherlands that holds the Guiness Book of World Records title as the largest: the Aalsmeer Flower Auction has a floor area of 990,000 m2. An interesting sidenote: Aalsmeer is also home to Endemol's global headquarters. You might be familiar with some of the TV shows they are responsible for.

All of these numbers are in 2-dimensions. Looking at volume, the Pentagon isn't even the biggest US Government building. The NASA Vehicle Assembly Building is larger than the Pentagon by about. 1.6 million m3. However Boeing's Everett Plant (where the 747, 767, 777, and 787 are all produced) dwarfs both of these structures at 13.3 million m3.

(Wikipedia article on world's largest buildings by footprint, size, and mass plus some special categories. There's also an intersting article about the PENREN.)