Editing increases file size?

I trimmed an mpeg (20 min. down to 16 min.) and the file size increased like 3x and when I converted AVI to mpeg-SCVD extension it also increased. I used WINDVD and Blaze video editing software. Why?

1. You probably recompressed the file in the process. The right way to do it is to do clean cuts without recompression. Use the 'merge & cut' tools under file, MPEG tools in TMPGEnc (http://www.tmpgenc.net)
2. Your source avi likely uses a high-compression codec like divx 5, xvid, or windows media video 9. MPEG-2 (SVCD) doesn’t have as high of a compression ratio as more modern codecs.