WTF ?! Fan :P

one fan is 200mm and one is 120mm, the bigger the fan the more air it will move.

a 200mm fan at 400rpm can move 2.5x as much air as a 120mm fan at 1200rpm*

* from xbitlabs.com

Larger fan blades (200mm vs. 120mm) move more air, even at the lower RPMs, thus the "cooler" comparison.

Now that being said, you can only install the size of fan (unless you custom cut your case) that your case supports. So, if your case only supports installation of 120mm fans, you can't install a 200mm fan instead.

What make/model of case do you have and how many fans do you currently have installed in it?

cases like the HAF 912 give you the option of 1x 200mm or 2x 120mm up front and up top

Understood and agreed. Still need to know what make/model of case the OP is using. Good point, nonetheless.

i wanna have a 922  

Nice choice. Also keep in mind that since the larger fans typically turn slower, they tend to run quieter, while moving more air. This is a good thing. Good luck!

Thanks   and thanks for your guys time  

Airflow is measured in cfm (cubic feet per minute), thus, a larger fan which contacts more air can move more air thru a larger opening at a slower rate.