I explained the limits of my investigation. Of course, given that the techniques of encoding are different, I expect differences in size.
However, I also ran the resulting files and visually appreciated the differences (or lack of).
And yes, some of the results are puzzling !
But I re-ran the tests and the results were confirmed (I run Windows 7 Ultimate. Figures from file Properties).
In relation to my Conference series:
I now have 2 DVD's which I find not significantly visually different when run on my PC (37x23 cm display)
1) DVD+R DL (8.5 GB)
Burned using SP quality level (Standard Play)
Roxio estimates: used 6,771 MB, free 1,768 MB, time left 36:55
Works fine on my computer. As expected, does not play on my player+TV
2) DVD+R (4.7 GB)
Burned using EP quality level (Extra Long Play)
Roxio estimates: used 3,593 MB, free 1,107 MB, time left 43:38
Videowave crashed near the end, but the CD is complete and usable: I just watched it on my player+TV