Can anyone give me any advice here

Windows XP / AMD 2600 processor

If I convert a divx / xvid (.avi) file over to .mpeg file redy to put onto dvd and play it back it tells me the mpeg playtime is around 1/4 of avi playtime (e.g 2 hr playtime .avi = 30 min playtime .mpeg)

I have used aare converter and showbiz but get the same results

Any ideas ???