The User Migration Package (UMP) file is first transferred to a temporary location on the hard drive of the client computer it is being restored to.  As a result, you need to have approximately twice the amount of hard drive space on the client computer of the UMP file you are restoring.  For example, if your UMP file is 4 GB, you would need approximately 8 GB of free space on the client hard drive in order to successfully restore the package. 

Note:  The amount of free hard drive space required to restore a UMP file is also dependent upon the compression level used when the UMP file was created.  If the UMP file is compressed, you will need enough hard drive space to allow for decompression of the UMP file.