[X4U] FileVault requiring WAY too much drive space?

[Trade Sniper]

I have a 57GB (primary) hard drive in my QuickSilver.  I also have a  
128GB secondary drive.  I'm trying to turn on FileVault for my home  
directory on that primary drive.  Right now, with an empty trash can,  
I have almost 39GB free.  I went to turn it on and I get an error  
messages saying that it needs an additional 4059.9GB free to turn on  
FileVault, to try deleting some files or emptying the trash.  4GB I  
could see, even 25GB, but not 4TB.  How can it require more that  
twice the space thats currently in use?

I know it needs free space, I could see it needing as much room as  
whats currently in use to make a copy, maybe even a little larger to  
account for the encryption, but 4TB more, for 18GB worth of used data  
can't be right?

The trick setup would be to copy and encrypt a little, then delete  
what it just copied and encrypted, then grab the next chunk, then it  
wouldn't need much free space to do the conversion.  But I have no  
idea what it does behind the scenes during this encryption conversion  
and this method probably wouldn't work or wouldn't be safe.  It  
probably needs to build it all then run some comparison against the  
whole thing to make sure its valid before it will delete anything.

Anyway, anyone have any ideas why it would require this much space?   
Or if there is some other problem?