Player displaying incorrectly

Hello there

I'm experiencing a problem with the player when I publish - I'm getting some extra 'edges' to either side of the project, which I don't want. It's the grey bits shown in the 2nd screenshot below - they don't show when I preview, just when I publish. I would like it to look like the first screenshot shown below. I can't see any settings different to my other projects, which all look as I would like them to!