DVD9 (Dual layer DVD) can hold almost double video content of normal DVD. DVD9 is 8.5GB, whereas normal DVD is 4.7GB.
Usually video of 180min and above, uses DVD9 or sometimes with 2 normal DVD's. There were titles, that we wanted to keep it 'originally' came in DVD9 without compressing it. Also, you can enjoy 1 DVD9 disc instead of changing 2 DVD's disc.
If title stating 'DVD9-compressed' means it was compressed from DVD9 to
normal DVD, therefore, video quality decreases.