There's no project file as the source. This has been done from scratch in Final Cut. My source video is a Prores422 (HQ) encoded using Mpeg Streamclip and added to the timeline via 'reconnect media' rather than standard import/drag and drop (as I was already half way through editing with a DVCPRO50 file when I changed to Prores)

Worth noting is that DVCPRO50 was also exactly the same when I started editing with it (already letterboxed even before ticking 'anamorphic 16:9) so there must be something left over from when i worked with that file that's forcing my prores file into this squashed version. What's causing it though, I have no idea :(

The original prores file is not letterboxed, nor is it letterboxed when dragged into the timeline manually, as you can see from the pics. So why is it letterboxed just because of the METHOD in which I added it to the timeline?

I'm so frustrated. I can't really move on with the project (compositing, rotoscoping, etc) until everything's the correct size.