bailes, it seems that WET will not transfer data from a 64-bit install to a 32-bit install. Since you are going the other way, there is no restriction and the problem is not the bit-ness.
Just to confirm, on the Map User Accounts tab, it is showing:
bailes => bailes
and on the Map Drives tab it looks something like this:
C: 40 GB selected => Windows7 (C:) 132 GB available
And the "Windows 7 (C:)" to the right of the arrow is grayed out. Is that right? Can you see that the amount of selected data is less than the available data? Are there any other drive mappings shown (e.g. D: => D: or E: => E: etc.)?
If the mapping is correct, that 40 GB of the "old" data are being restored to the 132 GB of available space, then something else is wrong. Do you get a Transfer report file? Right next to the shortcut to Easy Transfer in Start -> All Programs -> Accessories is a shortcut to the reports.
Somewhere in my travels, I read a thread where an MS employee asked a user with a .mig file that they couldn't open to send the file to him. From the way I read it, I believe that the .mig extraction tool available on bittorrent is a MS internal use only tool that has been leaked. I agree that they should make an extractor available to the general public. Even easier, they should use a standard compression format (zip, rar, 7z, etc.) so that folks could open their own .mig files and MS would not have to support an extraction tool.