In QuickTime Player, with your file open, press command and J keys to get the Properties window. Click the entry for Video > Visual Settings. The checkbox for High Quality is at bottom right. If you want this to be always enabled, go to QuckTime Player in the menu bar then, Preferences > General. Click the checkbox for “Use high-quality video settings where available”.
Also bear in mind that FCP is showing your video at a lower resolution to better enable real-time playback.
Nick