I must assume that you are asking about an image backup.
An image backup cannot be restored to a drive smaller than the one that the image was created from.
An image backup contains data required to restore every cluster within the partition.
All empty space on the backed up partition is compressed to almost nothing.
Therefor, the size of the backup is meaningless.
Restore of an image backup recreates the entire partition.
Restore has no way to eliminate the empty space in order to create a smaller partition.
I hope this helps.
That’s all for now, eewiz.