I don't know of a way to achieve what you're trying to do - that is shrink the 20gb log component of the backup before restoring, or tell restore to shrink it on the fly.

However, if your disk space is at a premium, the following may be worth a try. I've just tested it with a much smaller database and it worked ok....

Create a new "compressed" NTFS Folder.

Execute the restore, using MOVE option to direct the DATA component to your normal data directory, but the LOG component to this compressed folder. It may fit.

In Windows Explorer look at the properties of the restored log, it's "size on disk" should be considerably smaller than its reported size.

Detach the database.

Delete the LOG file.

Attach the database, specifying only the DATA file. A new LOG should be created in your default log directory.

Delete the compressed folder (and don't you dare use compressed folders for database files again! )