Suppose that one day I ate really a lot of sugar and junk food. So I made a high surplus of calories. Are all of those extra calories going to be stored as fat? Or does the body have a limit for that?
It's a tough answer, and it's based around your endocrine system. Your body releases insulin to direct fat cells to increase their storage.
If you do not have an endocrine disorder (such as diabetes), your body will become more insulin sensitive as your body fat drops, therefore you will readily store more fat. The more insulin resistant you are, the less fat you will store.
Which sounds awesome except for the part where all that gunk is floating around in your blood stream, causing plaque.
Also, depending upon the food you ate, as much as 10% of the calories are simply excreted.
I disagree with the idea that the body has an unlimited capacity to store fat. If that was the cause, there would be no plaque and no strokes, because all fat would be readily absorbed into fatty tissues.