Dual layer disks cost a whole lot of extra money, so the developers will want to keep the game somewhat small to keep the prices at a minimum.
It's not because it's cost a whole lot more to you that it costs a lot more to them.
When you're printing batches of 100 000 copies, the difference must be minimal between DVD5 and DVD9.
I would think that would be the opposite. Yes, they'd get bulk discounting, but if you have a 5 gig game it'd probably be more economical to cut something down to fit it in a DVD5 than to pay for the DVD9 disks. Since they are more expensive, those costs over 100k+ copies adds up.