Anime fansubs are almost always encoded with high-compression video codecs like DivX, XviD, h.264, etc. If you're trying to convert them to DVD-compliant MPEG-2 video that will play on a standard DVD player, you're going to see a jump in file size. Even tweaking your encoder settings, you should expect to use about 500-600MB of space for a 24-26 minute episode while maintaining video quality.
If you're just trying to archive the files on DVD to play on your computer later, you should be able to fit 4.38 GB of anime on one single-layer DVD. Just burn them as a normal data compilation.