Shrinking volume of HP laptop prior to restore

I have an HP g4 laptop with a 1 TB hard disk that I am trying to restore to an HP G62 with a 500 GB hard disk. 

The C drive that I am attempting to restore has 377 GB of used space on the 1TB disk. When I try to shrink the C drive in ShadowProtect's Mount and Dismount utility in the recovery environment, I get the Volume Size down to:

495 GB, of which 351 GB is used space.

That is still too large to fit on the C drive of my G62 laptop, which is:

450 GB, of which 342 GB is used space.

It seems to me that I should be able to get a 1 TB drive with only 351 GB of used space on it to fit on a 450 GB drive. Why is SP, when shrinking the volume, only getting the Volume Size down to 495 GB, with 351 GB of used space? Isn't thremaining 144 GB just blank space that SP should be able to delete to get the Volume Size down to 351 GB (or at least lower than 450 GB)? 

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