Why both videos are of same file size?
As it seems both videos have a bitrate of 118 (117) megabits per second.
This means - regardless off any other parameter - your video will take up 118 mb for each recorded second.

But as you already noted, a video with double the FPS should have double the data, so if you originaly exported the 4k60 video with 118, you can export the 4k30 video with 57mb/s and gain about the same quality.
There's more to it like diffrent codecs optimized for diffrent types of videos or Interframe Compression, so you can compress high FPS a bit easier. It is recommendable to go slightly above the half.

Keep in mind, that many consumer cameras tend to have a low bitrate for high FPS as the camera is not able to record more.