A +R DL dvd is nominally 8.5gb, and actually holds about 7.96gb. A single layer dvd is nominally 4.7gb and holds 4.38. You can see that if these ISO files are very close to the limit of the size of a single layer dvd, they will not fit onto a DL dvd even when you reauthor. They might need compression as well.
Re-authoring is not very time consuming if you have the right tools. Re-encoding is much more so.
Just burn them onto individual single layer dvds.