Well that depends on what tutorial you look at really! For example in the blackout set there's an 11minute long 29MB video and later on a 24min long 25MB video! Your audio can go down to 2kb/s, and as I said before change the screen colour to 16-bit - this has quite a large effect.
3D-Palace also has the help of a guy who's really good at compression, so I think that's another trick used!