Win7 volume shrink with unknown reason

I just had my MBA upgraded to Lion 2days ago,and installed Win7 ultimate, set the size at 40Gb. (my hard disk is only 125Gb)
But now,my Win 7 partition became 27Gb as shown in Win7, its also the same appeared 27Gb in the Win7 partition.
In the recovery ( by pressing option key during start up), it still showing 40Gb.
Wondering what is the matter.
Can anyone advise me on this?
Thank you.

Open Disk Utility from within Lion (booted to Lion) and tell us what it says about the BC partition. If you can paste a screen shot here that would even be better.

Disk Utility says your BC partition is 40 GB and that you have used over 25 GB. I'm not sure what you mean by it's shrinking every day? Did you actually install 25 GB worth of programs and data in Windows 7?

Windows and OS X do calculate hard drive sizes differently - OS X measures it using base-10 notation and Windows uses base-2 notation. Regardless, that doesn't explain the size difference. If I did the math properly, 40GB would be 37.25GB in base-2 notation (or how Windows would see it). So that's not it.

Can you provide a screenshot of an explorer window from Windows showing your HD?