There's a difference of roughly 7MB.
Strangely enough though for single layer the -R variant is the larger whereas for DL, the +R variant is bigger.
Here are are the exact sizes in bytes according to the DVD specifications.
DVDâˆ’R SL 4,707,319,808
DVD+R SL 4,700,372,992
DVDâˆ’R DL 8,543,666,176
DVD+R DL 8,547,991,552
In case anyone wonders why those figures all seem rather large the sizes quoted in DVDFab are actually expressed in mebibytes (MiB) rather than megabytes (MB).