60fps is a little tricky and some adjustments may be required.
First thing is to make sure you are using "High Efficiency" and not "Most Compatible" for the video format.
If that doesn't help, try doubling the "Avg bitrate" and then decreasing until you get something that is acceptable.
Remember, bitrate is tied to quality but is also directly tied to file size so the smaller the bitrate the smaller the file size.
There are also reports of "focus hunting" when using 60fps so be sure to manually set the focus (tap and hold on screen until you see "AE/AF Locked". Tap and hold again to turn this off).
Tags: 60fps, settings, videos