Probably because it images the entire partition, and that's what its copied back. Have a look in Disc Managment and see if there is any unallocated space. It might just need a drive letter allocating.
When restoring an image there are a number of options available, one of which is the partition size you are restoring to. Part of the restore wizard is an option to resize a partition, just by dragging its borders. Doing this should have left you with a separate partiton for the lost 40gb.