Why my desktop becomes bigger

i think when you play your game you use lower resolution than your desktop so when you quit your game your desktop probably stays at same resolution of your game or it switches back to the original resolution but your icons will be in different places... cant think of a solution other than play your game at same resolution as your desktop... if that is even your really your problem... not big expert but i think its that
 
After you've finshed playing the game, right mouse click in your desktop, screen resolution, and put it back to it's former resolution.

Try running the launcher for the game, it's found in program files, under the producer, or name of the game, when in the launcher, adjust the resulution of the game. Since I don't know which game my help is limited. Another way is to start the game, go to options, or settings, grapgic options or screen options, and set the resolution. Resolution in games most of the ltimes looks like
(Series of 3 or 4 numbers)x(Series of 3 or 4 numbers) x (Amount of Herz)
e.g --> 1280x1024 x 60 herz
Look for this option and adjust the game's resolution to the same as the desktop resolution.

Hope this helpes