I was having the same problem with FCP 5. I was exporting an 18 minute sequence with NO effects and it was 28GB and took 22 mins to export a “reference” file WITHOUT selecting “make movie self-contained”…
Not sure if it’s the same problem, but I found that I had the sequence settings set to 8-bit uncompressed and the capture settings set to 10-bit uncompressed, and even though nothing needed rendering, I think it was automatically converting from 8-bit to 10-bit, and therefore exporting a large file rather than a reference file. I looked at all the settings and made sure they were all 10-bit and did it again and this fixed the problem.