I just did:
ffmpeg -i inputfile -r 30 -vcodec libx264 -an -s hd480 output_30.mp4
ffmpeg -i inputfile -vcodec libx264 -an -s hd480 output_60.mp4
on a 60fps file.
Why is the second file 27 MB and the first file 49 MB despite the fact that the first file contains only every second frame?