Out of curiousity, I was doing a file-size comparison test on the Osmo Pocket where I take exactly 1 minute of video (in Auto/Fine mode) in 1080/30, 1080/60, 4K/30 and 4K/60.  As expected, the 1080 clips came in at 261MB (for 30fps) and 590MB (for 60fps).   No mystery there...about what I expected.

However, much to my great surprise, the 4K clip file size difference was negligable, regardless of whether I was using 30fps or 60fps (see attachment).  

4K / 30 = 738MB test 1, 739MB test 2
4K / 60 = 741MB test 1, 737MB test 2

As you can see, in my 2nd test, the 60FPS clip was actually SMALLER by 2MB, even though I used the same subject with nearly static content for both clips.

Does anywone know why there's not a bigger difference in the 4K60 file sizes vs 30fps?   I double-checked my settings (and the clip properties) to ensure that my results were accurate...

You should use a program like MediaInfo to look at the files and find the bit rates and other relevant recorded data.
Look here
Use the tree view and you may find the overall bit rate is smaller.

Hi.

I'm not sure what settings you've used when in 4K but if you're recording at the maximum bitrate then regardless of the frame rate the bitrate will still be the same and therefore the file size.

I don't shoot 50fps but use the max settings at 25fps, which I believe is a VBR with an average of 100Mb/s If I switch to 50fps the bitrate will still be the same as 25fps but each frame will only be half the bandwidth/size of the 25fps version.

Cheers,
Dave.

Yes, in 4K, 30fps and 60fps are both 100Mb/s, so it is perfectly normal that file sizes of the same length clips are the same. I don't know if it is a hardware limitation or they just did not want to narrow the compatible card range further more...