gb.and

i want to copy a original movie DVD with my sony dvd writer....but i fail because....the DVD i want to copy is of 8 GB.and i have a blanck DVD of 4.7 GB.and to my surprise the 8GB DVD is single layered.how can i write 8 GB movie on a 4.8 GB DVD.every movie DVD i want to copy is around is 7.5...