Q: Compressor making no difference in reducing file size between 25fps and 50fps

Hi

I'm using the

Video Sharing Services>HD 1080p

built in compression settings.

Video length 6m06s.

Ive got footage at 1920x1080p 50FPS that at full res export from FCPX is 10.96GB - Compressor takes it down to 971.8MB

Ive got the exact same video but at 1920x1080p 25FPS that at full res export from FCPX is 5.53GB - Compressor takes it down to 946.7MB

So its only saving 25.1MB even though its half the frame rate

Surely this isnt right?

Can anyone help at all?