To go along with what ignatzatsonic and xxxdannyxxx said.
How long have you had Win 7 installed?
I think the Windows folder grows in size over time, especially the winxsx folder.
Do you have a lot of programs installed?
Do you install/uninstall a lot of programs?
Here are 2 Win 7 x64 PC's folder sizes (size on disk) i just checked.
Win 7 was installed apx. 3 years ago.
PC-1 is used a lot more than PC-2, and has more programs installed.
PC-1
Windows folder 25.6 GB
winxsx folder 13.5 GB
PC-2
Windows folder 21.3 GB
winxsx folder 8.55 GB
These folder sizes don't cause any issues for me.