Are you talking about the dvd's you buy in the store or something else? I know for a fact that even if there's only one movie on a dvd that you buy from the store, it is at least 8gb and is a dual layer. It's technically called DVD-9 format. They do this so you can't just drag the files on the dvd to a blank one and sell them!

It's still pretty easy to convert a DVD-9 to DVD-5 so you can copy a dvd, but I won't go into that here.

: Decrypting the dvd's are easy so long as you know the format, but yeah, they're DVD-9 format on regular ones. I haven't seen any disks bigger than 8.7gb(except blu-rays) so unless it's a blu-ray, it's still just a dual layer disk.