Buffing and playing right away large mp4 file.

Hi, I have a regular server that play movie. I use JWplayer. When it's small file the player play them right away, but when it's big file he does not play them right away. I would like to fix that matter. Is it related to the server or the player? Let me know what you can do to help me out, don't really mind what you acheive in the past. If you can come out with an idea to fix it you will be taken. Thanks, Alain