Captain America made me realize the Ark was essentially a large Autobot flying wing bomber. Germans had flying wing bombers during WW2 that didn't get pass the drawing board stage. This pretty much tells why the Ark was designed that way. It was designed to deliver the pillars. On another note the Decepticon Motherships take their concept from airship carriers. Like the USS Akron. The Xantium is a big rocket ship is obvious.