If the snack has anything that raises insulin levels (high glycemic carbs or specific amino acids) insulin levels surge. Excess calories from the snack get locked away in fat cells, and the machinery involved in allowing cellular fat to be mobilized is locked shut tight till insulin levels are allowed to drop (which in the vast majority of adult North Americans is never).  So literally, a moment on the lips, a lifetime on the hips.  
Calories in calories out is the least important piece of dietary metabolism. 
The macronutrient source of calories and the duration between feeds is by far a more important aspect.