>>/2062/
Thats about the average size of all the ones I've downloaded. I usually use yt-dlp since it uses ffmpeg in the same way but does a second pass on the file for mp4 container optimizations like moving the moov atom to the front, giving it a structure like this

$ ./devel/copyclips/showboxes.scm /tmp/kane.mp4
ftyp 32 bytes at pos 0
moov 1110034 bytes at pos 32
*mvhd 108 bytes at pos 40 0 over minimum
*trak 894707 bytes at pos 148
tkhd 92 bytes at pos 156 0 over minimum
edts 36 bytes at pos 248
*elst 28 bytes at pos 256 -4 over minimum
mdia 894571 bytes at pos 284
*mdhd 32 bytes at pos 292 0 over minimum
*hdlr 45 bytes at pos 324 12 over minimum
*minf 894486 bytes at pos 369
vmhd 20 bytes at pos 377 0 over minimum
dinf 36 bytes at pos 397
*dref 28 bytes at pos 405 0 over minimum
stbl 894422 bytes at pos 433
*stsd 194 bytes at pos 441 15 over minimum
*stts 274352 bytes at pos 635 274320 over minimum
*stss 2352 bytes at pos 274987 2336 over minimum
*ctts 193640 bytes at pos 277339 193624 over minimum
*stsc 179392 bytes at pos 470979 179376 over minimum
*stsz 137192 bytes at pos 650371 137172 over minimum
*stco 107292 bytes at pos 787563 107276 over minimum
*trak 215114 bytes at pos 894855
tkhd 92 bytes at pos 894863 0 over minimum
edts 36 bytes at pos 894955
*elst 28 bytes at pos 894963 -4 over minimum
mdia 214978 bytes at pos 894991
*mdhd 32 bytes at pos 894999 0 over minimum
*hdlr 45 bytes at pos 895031 12 over minimum
*minf 214893 bytes at pos 895076
**smhd 16 bytes at pos 895084
dinf 36 bytes at pos 895100
*dref 28 bytes at pos 895108 0 over minimum
stbl 214833 bytes at pos 895136
*stsd 123 bytes at pos 895144 -56 over minimum
*stts 24 bytes at pos 895267 -8 over minimum
*stsc 40 bytes at pos 895291 24 over minimum
*stsz 107296 bytes at pos 895331 107276 over minimum
*stco 107288 bytes at pos 1002627 107272 over minimum
*sgpd 26 bytes at pos 1109915
*sbgp 28 bytes at pos 1109941
*udta 97 bytes at pos 1109969
meta 89 bytes at pos 1109977 0 over minimum
free 8 bytes at pos 1110066
mdat 2882851964 bytes at pos 1110074
$ stat /tmp/kane.mp4
  Size: 2883962038      Blocks: 5632744    IO Block: 4096