What does "fattening" mean?

The word fattening means, "to make fat."  What is much less clear is how that word is applied. 

For example, orange juice has NO fat.  It is a fat-free food.  So are bananas.  Fat-free, natural, containing NO fats of any kind - no saturated, polyunsaturated, or monosaturated.  No fats.

BUT, both those foods can be "fattening."  How is that possible?  Both orange juice and bananas contain a large amount of carbohydrates.  They are the so-called complex carbohydrates found in plants, but in the "wrong" environment, carbohydrates of any kind can be "fattening."

Here's what happens when you consume carbohydrates.  The body breaks down the molecules into sugars.  Sugar in the blood stream invokes the production of insulin.  Too much sugar in the blood stream results in the storing of the excess as glycogen (sugar, a carbohydrate)  in the liver. 

But how does that make us fat?