MacBook Pro - How to re-allocate memory between Windows & MAC OS?

I am struggling to reallocate the memory between the two operating systems on my Macbook pro. In the first place, I set 170 GB memory for Windows and 80 GB for MAC OS. A weird thing happened is whenever I go to virtual machine to access to Windows Vista, the shared memory goes down even though I don't install or upload any new file on my MAC OS. Now my Macintosh HD has only about 30 GB memory . So I have 2 questions:
1. Why does it happen? Is there anything wrong?
2. How can I reallocate the memory between 2 OSs? ( E.g. 170 GB for Win and 80 for MAC)

Thanks for your help.