Dual-layer DVDs could actually fit as much as 9GB per side. For example, Super Smash Bros Brawl on Wii came on a dual-layer DVD, but mostly because they couldn't fit the ridiculously long intro video and the story mode cut scenes that were pretty generic and added nothing to the game, in my opinion.
Regular Blu-ray discs can fit up to 25GB per layer, so 50GB for dual-layer discs. They've also figured out how to stack even more layers onto a disc, so theoretically games could reach over 100GB per side.
To me it's all a gigantic joke, because you don't need to waste that much space to make a fantastic-looking game, which has been proven by many developers time and again.