Interesting. I just exported a ProRes422 version (2.67GB) of my project from FCPX and used HandBrake to convert it to MP4 file:
Handbrake “High Profile” MP4 version = 294.6MB
FCPX Share>Computer>Quality MP4 version = 500.6MB
I presumed the Handbrake version would be larger, but I was wrong.