Lara, I'm not a MAC user, but it strikes me that the 7.96G you can record is the same amount of space as DVD-DL (dual layer DVD) discs can hold. Is there any chance you really bought DVD-DL discs instead of BR discs? Worse yet maybe you got a mislabeled package?
(I know that DL DVDs are labelled as 8.5GB, but the manufacturers mean 8,500,000,000 bytes, when your computer would mean 9,126,805,504 bytes. So, your computer will see 8,500,000,000 bytes as 7.96 GB.)
Let us know what you find.