為何我用Nero隻能燒兩個兩個電影在一個DVD 盤中?

I am using Nero to burning DVD , but I don’t know why when I added only two AVI files then I got a message which said that I already used about 4.7 G space. I believe there two files‘s size are about 1600 MB. I know some people can put about 6 movie on one DVD but I couldn’t I hope if someone can tell me why.