You can't put more information onto a disk than it will hold.
Think about it, you have a pot that holds 4.7 cups of water and you have 7.5 cups that you need to put into a pot. You can't fit it into the 4.7 cup pot of water. Won't work.
Unfortunately, 7.5 GB is too much data for a dvd that can only hold 4.7 GB. Although, what you need to get is a double layer DVD, which in fact holds up to 8.5 GB of data. They are usually sold as DL DVD-R or DL-DVD+R. Any kind will do. Hope this helps.