Why Does Insulin Cause Weight Gain?

There are so many books out there on diabetes reversal. The more I read the more I am always surprised. In his book The Obesity Code Dr. Jason Fung, a nephrologist from USA is speaking about the obesity epidemic. 

Back to the point - things that surprise me. 
We are all raised to believe weight is calories in vs. calories out. "But if that were so, why did the medication I prescribed - insulin - cause such relentless weight gain?"
And that is so true. Why does insulin cause weight gain? People don't eat more on insulin, they eat the same amount, but they gain weight. Why does that happen?

"The conventional view of obesity as a caloric imbalance did not make sense. Caloric reduction had been prescribed for the last 50 years with startling ineffectiveness."

Welcome to my world. As I read more and more I get more and more astounded by what impact nutrition has and what little we know about nutrition. Our science and discussion is still rooted in old dogma, calories in and calories out; but the human body is so much more complex than that.

If you have any theories on why insulin causes weight gain feel free to post it in the comments below.

Why Does Insulin Cause Weight Gain?