Tail heavy
Consider a plane that is very nose heavy - think of hanging a big container off the nose of a flying aircraft.
To continue flying straight the plane has to compensate for this nose down moment. The way it works is that the plane is retrimmed to keep the balance and push the tail down (with the stabiliser) - This causes drag and increases fuel consumtion.
Now move the container just to the front side of the wing - here it causes a lot less moment that the tail has to compensate for - less drag = less fuel
There is other aspects to this but I hope this explains the question in an easy way without sounding too stupid..