I finally setup my computer for BitLocker™ Encryption today and I was surprised to see that volume was full as soon as I hit "Turn on BitLocker", I had roughly 6GB of free space left and rest all was gone!

Don't worry, No need to hit panic button!
It seems this is how BitLocker™ solves the problem of determining whether to encrypt or wipe the unallocated chunks from your hard disk by creating a huge wipe file. More details on this can be found on BitLocker™ Drive Encryption Team Blog.

How can you get this unallocated space back?
When encryption of the volume is paused or completed, the wipe file is deleted and the amount of available free space reverts to normal.