The kbps figure is the bitrate...in this case you are looking at 448kbps vs 128kbps (kilobits per second). The smaller avi is using a very much smaller bitrate. Which is one reason it is not as big.
The other reason can be seen in the resolution of the two videos. 608x256 vs 368x256. The first is nearly twice as big as the second horizontally.
Reducing both of these parameters can give you much smaller files. The bitrates are very low, particularly the 128kbps. What video codec was used in these? Xvid, divx, H264?