If you're trying to copy a DVD, then in theory shouldn't it fit on a DVD? Unless you're trying to copy a DL DVD onto a single layer DVD. If you're trying to copy from a DL to a SL, I don't think you'll ever be able to compress enough to fit it on there (And if you do, I give no guarantees that you'll still have uncorrupt data left at the end). Also, compression for data is done by zipping (Or whatever method you choose). Are you trying to copy a data DVD or a video DVD? Also, which program are you using?
Tell us some more about this copying please. Also make sure that you aren't trying to burn to a CD (In the toast settings)