The major difference between D5 and D9 DVDs is that D9's capacity is larger than D5's.
A D5 disc is a single-sided single layer. It holds about 4,700,000,000 bytes and that is 4.37 computer GB where 1 kbytes is 1024 bytes. On the other hand, a D9 disc is a single sided dual layer. It holds around 8,540,000,000 bytes and that is 7.95 computer GB.