BluRay and DVD Exports encoding at half of the selected bitrate

I have produced a 2-hour movie in FCP X, using Prores media.

I'm trying to produce DVD and BluRay files Using H264.

I have tried exporting directly from FCP and via compressor but the files produced are only half the size predicted, and the bitrates being used by FCP and Compressor are roughly half what I selected. As a result the video is of poor quality.

When I export .Mov and M4a files of the same movie I don't have this problem - the exports are the size and quality that they should be.

Does anyone know why this is happening and how I can correct it? It's driving me insane!