I use this and have dropped mine down by half or more and still come out 720P,
So changing a 4Gb film down to 700MB has been fine for me.
Just choose two pass by size and select the size you want the final to be. It does a nice job. My upload speed is terrible (.5) so it's helped a lot.
The app and all of the selections I use are in the picture. The app is free.
