TMPGenc vs WinAVI

I have a raw 140k file (30 seconds of footage) that I transferred from my Dig8 Camcorder. When I use TMPGenc to convert into MPEG2, the file size is about 40k. The same file encoded using WinAvi gets me to about 110k. Shouldn’t the two files be about the same size? From playback on my computer, the quality seems about the same.

Probably because different bitrates are used for each file.