I've always just kind of assumed nature optimized the way all things are formed for what they're designed to do. Apparently, however, this is not the case.
Though it's a myth that bumble bees shouldn't be able to fly with their rather small wings and rather large body, the way in which they do fly is incredibly inefficient. Essentially they move around the air through sheer brute force. Not only are their wings unsynchronized, but the way in which they flap i makes it impossible for air flow to aid the bee in traveling through the air more easily.
Its huge thorax combined with the high energy necture diet allow for the necessary force to create thrust with such tiny wings for such a heavy body. It's speculated that this brute-force method was developed so that bumble bees can maneuver through the air more easily, sacrificing overall flying efficiency, or as a result of the already wide body of the bee.
More on this can be found at: http://www.sciencedaily.com/releases/2009/05/090507194511.htm