Why does halving the frame rate with FFmpeg barely reduce file size?

I have some stream rips at ~2k kbps I’d like to cut down.

ffmpeg -ss 4:00 -to 4:10 -i in.mp4 -an -vcodec libx265 -crf 26 -preset slow 26s.mp4
ffmpeg -ss 4:00 -to 4:10 -i in.mp4 -an -vcodec libx265 -crf 26 -preset slow -vf fps=30 26sr.mp4

The first is 60 fps and 839 KB. The second is 30 fps and 807 KB. Why such a small difference?