The problem is that getting a smaller size encode takes time and it seems they are all in a hurry so they use faster settings that take more space for the same quality.
JAV is also more problematic than most movies since it's all interlaced content and a fast deinterlacer will also make the video bigger than if you use a slower one.
I usually get a final video of about 1/3rd the size of the original by using the highest quality settings I consider not to be a waste of space(meaning the closest to the original visually without taking more space than needed).
2.5GB for a 3 hour 720p movie is really pushing it to the limit though. I just finished encoding a 2 hour BD JAV(sadly not a lesbian one), the 720p version takes 3.7GB and the 1080p one 7.94GB. I could probably push the compression further than that since it's less noticeable at HD resolutions but I really dislike compression artifacts.
hehe and I though I was bad 
