All uploaded files are automatically transcoded to ensure the video can be played back in a webbrowser and on mobile devices. To create this video file, we have a default preset which is applied to all videos. This default preset will keep your original resolution, but it has a 1 Mbit/s bitrate, which may be too low for your video. You can change the bitrate for an individual file by recreating it (choose File > Recreate), or if you want to change the default bitrate used for all incoming videos, go to Control > MediaLab > Settings > Video.
For more information about bitrates, also see "What bitrate should I choose for my video?"