how do i compress video files so they're smaller but still have its quality?

am i using the word 'compress' right?

like for example..Ive downloaded a movie in .divx and its about 1hr 35mins long with about 700MB

then i download this other movie in .divx which is 50mins long with 700MB

how does that work?
thats why i wanna try to make the file smaller..
any ideas?