If you want convert video from big size to smartphone size use this Libav tool
Ubuntu:
apt-get install libav-tools
or Gentoo
USE="mp3 speex threads v4l jpeg2k aac amr cpudetection faac opus vdpau x264 webp x265 xvid " CPU_FLAGS_X86="mmx mmxext sse sse2 sse3 sse4_1" emerge -av libav
and command for convert
avconv -i original_video.mkv -b:v 600k -s 576x432 -strict experimental small_size.mp4