Install Handbrake. Go to preferences and uncheck "Use ipod friendly extension".
Click Dimensions tab. Turn off Auto-Crop and set "Anamorphic" to "Loose"
Load your obese video file and set the "Height" value to 720 or 1080 (720p or 1080p as desired)
Click "Add to Queue" and then "Start"
Compare the size of the result and be amazed!
There is much more you can do, but that will give you a good idea where to start. Some of the biggest niggers I run into with fatass video files is uncompressed video (MPEG-2/BD) in a mp4/mkv container, audio tracks in formats like FLAC, extra languages I don't need and motherfucking cocksucking PGS subtitles.