The size of the output file is determined by two very critical things: the length, and the bitrate. You haven't told us the length nor what sort of AVI you're rendering to, so we're doing a bit of guessing here. Is your video somewhere around 1 hour, 41 minutes? A DV-AVI of this length would be just about 22GB, which is a completely normal and expected result. DV-AVI has a combined bitrate of about 29.5Mbps (megabits per second). Most DVDs are around 4 to 8Mbps. So, converting DVD material to DV-AVI should make it somewhere around 7.5 to 3.75 times bigger.
What are your plans after you've joined them together? How and where do you want to show them, and on what media? We can help you suggest better ways to get smaller results if we know what you want to end up with.