[FFmpeg-user] [offtopic] expansion of video content

[JD]

I am at a loos what newsgroup to ask this of,
as google provided no relevant answers about
this problem.

I always back up my dvd's to HD in case they get
damaged, as I do loan them to friends sometimes.

At anyrate, I use dd to copy the dvd to HD.
On HD, the size of the iso file (after copying SOME dvd's)
is in excess of 20GB. such as 22GB+, 26GB+...etc

Could someone explain why this is?

I thought dvd's have capacities of 4.7GB, 8.9GB for double density.
I also know about dvd's that have slightly more than 10GB capacity.

So, how is it that the iso image can get that large?

Thanx in advance.