Vimeo 60fps conversion smaller than the 30fps conversion

[Michael McNabb]

When I upload a 60fps 4k video to Vimeo in either ProRes or H.264, Vimeo makes 4k, 2k, and 1080p versions in both 60fps and 30fps. However, the average bit rate (and file size) is always smaller for the 60fps version than the 30fps version. This seems wrong. Even with the best inter-frame compression, I would expect it to be 20-30% larger to maintain quality. So viewers get either lower quality or a lower frame rate, and can’t even choose unless they download. Vimeo support won’t give me a straight answer and just says it “depends on the complexity”. Am I right to think that their 60fps codec settings are too aggressive?