Why both videos are of same file size? I am using ProMovie Recorder to record high quality videos using iPhone X, and I noticed a weird thing today.

Here's what I did:


I recorded a 4K 60fps video for 6 seconds (HEVC encoding).
I recorded a 4K 30fps video for 6 seconds (HEVC encoding).
Both the above videos captured the same physical content.
Both videos were captured with the same bit-rate, ~118 Mbps.


Here's what I found:


The two videos I captured (with different fps) are of the same file size - 84MB. How? Why?






Question


  A higher frame rate video with same bit-rate as that of a video with
  lower frame rate should be double the file size, shouldn't it?
  
  Why isn't there a difference in file size? More the number of frames
  will lead to more the file size, since we're storing extra 30 frames
  for every second, right? Or is the underlying HEVC encoding optimises the file
  size for higher frame rate videos?
  
  Or, can slight differences in exposure cause significant file size difference?
  
  Please correct/clarify.
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.