I would suggest something like FFmpeg, though would set expectations on quality if you go from a 500MB file to a 50MB one – though a lot will depend on the amount of optimization already present in the source… I was able to get a 270Mb file down to 29Mb without visible loss of quality using the following:
./ffmpeg -y -i SourceFile.mp4 -s 1280x720 -c:v libx264 -b 3M -strict -2 -movflags fast start DestFile.mp4
The above will give you a 1280×720 output, at 3Mbps using h264 in an mp4 container, and will also do a second pass to move the moov element to the front of the file enabling it to start streaming faster. It will not re-encode the audio so will keep whatever quality you started with
You may want to play around with the frame size and the bitrate to get the filesize to match what you like/need