Mr. X is correct on this one, here is a simple analogy I'll explain more.
Say you have 10 tons of gravel to move to another location, and a 1/2 pickup truck to move it. Over the course of the day 20 loads will get it there. But some idiots says hey lets save time, throw it all on there. Not much of a truck left, correct?
Here is a bit more scientific approach. Your body can store a certain amount of glucose (the fule your body uses mainly to move) Sugars go there first, when thats full you body says hmmm what to do, ok we've got all this yumm energy to use instead of wasting it by not digesting and pooping it out, we'll digest and store it!
And it chooses fat to store it in.