It takes twice the file size to double the framerate without reducing visual quality of the original frames, whenever I try, with HEVC, AV1, etc. I can take a lower framerate video, run some software of constant size to interpolate the frames, and play back at a higher rate, but in practice I can't save that higher framerate video to a new mp4 file with small or constant size increase. It's got to be about linear.
Surely this is a limitation of the encoder or a configuration problem, as encoding a video with faster motion at equivalent quality ordinarily requires a higher bit rate, and from the standpoint of the encoder, I wouldn't expect there to be a difference between half the frame rate and everything moving twice as fast.
Are you doubling up frames for 25fps-in-50fps though? Because with long-GOP codecs like HEVC there ought to be just about no difference between pairs of frames. For sure with intra-only codecs twice the frame rate will be twice the file size.
