There are two main factors involve in reducing the size of mp4 video
i: width and height of video, greater their widths, greater will be size and slower will be streaming e,g 360p, 480p, 720p, 1080p
ii: video bitrate, greater will be their bitrate, higher will be quality, size and slower will be their streaming.
For faster streaming i recommend using
i: 480x360 or lower width and height of video ii: -b value 360k or lower. iii: -ab value 64k (optional)
Hope this will help you.