Actually, CDs hold 650 megs, or 742 megs for mode 2 (but uncommon).
DVDs come in a variety of formats:
12 cm single layer hold 4.7 gigs, double layer holds 8.54 gigs.and
8 cm single layer is only 1.46 gigs, double layer holds 2.66.
Of course, the PS2 can play both types of DVD, in both sizes (you need a special adapter for 8 cm disks in vertical mode), and can play mode 1 CDs.
When you buy a dvd movie, if it is widescreen on one side, tv on the other, it is a two sided single layer. If it is more golden, and has a label on one side, it is a dual-layer DVD. The dual layers are becoming much more popular now that costs for production have fallen. However, CDs still cost pennies to make, while DVDs are relatively expensive, dual layer DVDs costing at least $3.00 dollars still. That\'s why they release on CD if they can.