When you did the prepare, did DVDA warn you that it was going to compress the video? DVDA wants things as an MPEG-2 stream, so it's decompressing the mpeg-1 and recompressing it at a higher bitrate (probably the default of 8,000 kb/s if you didn't change the setting), hence the larger size.
I do agree with ScottW. On the otehr hand, why would you want to burn a DVD with an MPEG1? You're loosing time and money. MPEG1 quality is definitely poor.