I don't know why MM keeps crashing, but it is a bit "flakey". I have certainly had my fair share of crashes(most often when trying to add transitions). Try closing down as many background programmes as possible as this may help.
25GB does seem a bit on the large size. From recollection, I have seen on these pages that 4 minutes of .avi should be about 4GB. When I get home tonight I'll have a look at some of my files to test that fgure.
When you say "...upon viewing the 25GB file clips seemed to be scattered and duplicated..." I am not sure exactly what you mean. As far as I know, once you have finished the movie and saved it, you should end up with one file (admittedly large if its an .avi) in somewhere like "My Videos". Do you mean you have more than one copy of the finished movie?
It might be worth your while having a trawl through these click here and click here