Yeah it was the audio I realised – it was a single mono .wav file but Final Cut was converting it into stereo, hence the file size. The solution was to just export the video as a reference file and then add the audio to it in quicktime and save it, again as a reference file. It adds an extra step but it gets you there in the end. Don’t suppose there’s any way to set up a Final Cut sequence with a single mono audio output is there?