Post-Production : Converting from PC to Mac by Ramon Richardson

I finally got a Mac version 10. something. I have 1920 X 1080 editied phootage that I did with Windows Live. When I convert it to Mp4 Mov or Mpeg to be used with the Final Cut or Imovie the phootage loses alot of quality. How do I fix this? Thanks. Here is a trialer that I started on WIndows and Finished with a Mac https://www.youtube.com/watch?v=uiXjkWee__k