Any1 know How/If a Video van be compressed after its been shot

[VERYUNHAPPY0420]

I understand why it's difficult to send videos considering that cameras have so many megapixels which create huge files. 

Does anyone know, or use (even better), a video compression app, website, etc. that actually does what it says??  I have tried at least 7 apps and none do what they claim they will do.  I also read our heard that Android phones have no way to compress the file size once the video has been shot.  I know I can change the settings and take the quality of the video down and it would lower the size of the video.  But, I'm referring to completed videos.  Taking a video, compressing it down to a smaller format which enables the video to now be shared.

Thank you in advance for all help.