Question: DVD Sizes, What is DVD-5, DVD-9 and Double Layer DVDs?
Double Layer (sometimes called Dual Layer) and Double-Sided DVDs each hold more than the standard 4.7GB of a single-sided single layer DVD. Many new DVD Copy softwares for Mac users burn Double-Sided and Double (or Dual) Layer DVDs. Learn more about the different DVD sizes below.

Answer:
DVD-5 holds 4.7GB and is supported by the DVD+R/RW and DVD-R/RW formats. It is also known as Single-Sided Single Layer.
DVD-9 holds 8.5GB and is supported by the DVD+R and DVD-R formats. It is also known as Single-Sided Double Layer (sometimes called Dual Layer). The official names are DVD-R DL and DVD+R DL

How to Accomplish DVD9 to DVD5 Compression for Mac users

Maybe you have a nice dual layer DVD movie, and you want to backup it for sharing or conserving. However, you just have some DVD5 discs in hand now. Then how can you accomplish your goal according to what you have? Just check the following guide that helps you finish copying DVD9 (Dual Layer) movies to DVD5 (Single Layer) on Mac.

Now what you need is just a cool tool Mac DVD9 to DVD5 Copy , which is easy operation but has more functions than iDVD.

More information from here : How to Copy DVD9 Movies to DVD5 Movies on mac