Using a G5 iMac with OSX.3.9

I have converted some .avi files (tv shows) to mpg. The problem is that as avi files they were about 350-450 mb. As mpg they are 1.95gb. My son has told me that this is because there are a lot of un-needed files attached to the mpg. However he doesn't know how to find or get rid of them on a Mac (he's a PC user). Help anyone??

Second problem. When I convert some avi files to quicktime using a AV to QT utility I sometimes get a message telling me that the audio is in an unknown format and therefore the converted file will not have any audio--not a good thing for watching later!! Again any help will be appreciated. And "No" I do not want to change from the Mac to a PC!!!