DVD burning question

[WelshWitchPMD]

I just bought a new DVD copy program. It is called 'Easy DVD Copy" by Roxio. I have some DVDs that I was not able to copy with my other software. I would put the DVD in to copy and it would read the data and then when I put the blank DVD in it would tell me that the source did not match.

With this new program I put the DVD in to copy and a message came up saying that the DVD that I was trying to copy was too large to fit on a regular DVD, It asked me if I wanted to compress the files and remove all other content? Will that change my original DVD in any way? What will the DVD that I copy be like? It is a concert if you know what I mean.
What I don't understand is how can 2 DVDs that are both 4,7 GB not be the same size?