De Shrink?

OK you can shrink a dvd to fit on a single layer dvd but can you do it in reverse?

That is take a single layer dvd that has been shrunk and restore it to it’s original unshrunk form and put it on a DL dvd?

^ No.

Obviously No.
But why dont you just simply use the original movie, and then you can do whatever you want with it (SL, DL, or whatever).

Transcoding/reencoding a DVD is a lossy process, meaning that during compression, you lose information that you can’t get back. This is different from some other methods of compression that are lossless (such as zip or rar), where all of the information is retained and you can extract the original information by decompressing it.