What exactly affects gas milage?

I've hear alot about gas milage, and its always said that the larger engine and heavier car is the one that gets less mpg, but can this be accpeted as a fact? I see v6 blazers that get 15mpg and under with a weight around 4000-4500lbs, then see v8 dodge durangos around 5000lbs that can easily get 18-20mpg. So I come to the question, what exactly are the true factors on MPG?

there are MANY MANY things that'll make you burn more or less fuel. basicaly, anything that moves you from A to B less efficiently will make you burn more fuel. just to name a few: aerodynamics will affect economy (especially at high speeds), weight will affect economy under acceleration, anything that makes your engine less efficient will burn more gas, a larger engine (given the same efficiency) will burn more fuel than a smaller engine, friction and weight in the drivetrain will lower economy, etc. etc. etc...

Also, things like the age of an engine will affect fuel economy. If you have two engines of identical displacement and horsepower, but one is new and the other was designed 20 years ago, the newer engine will have better mileage because of improved modern technology.