$ yt-dlp -F 'https://www.youtube.com/watch?v=xC5uEe5OzNQ'
  [youtube] xC5uEe5OzNQ: Downloading webpage
  [youtube] xC5uEe5OzNQ: Downloading android player API JSON
  [info] Available formats for xC5uEe5OzNQ:
  ID  EXT   RESOLUTION FPS │    FILESIZE  TBR PROTO │ VCODEC       VBR ACODEC      ABR     ASR MORE INFO
  ───────────────────────────────────────────────────────────────────────────────────────────────────────────────
  sb2 mhtml 48x27          │                  mhtml │ images                                   storyboard
  sb1 mhtml 60x45          │                  mhtml │ images                                   storyboard
  sb0 mhtml 120x90         │                  mhtml │ images                                   storyboard
  139 m4a                  │     1.55MiB  47k https │ audio only       mp4a.40.5   47k 22050Hz low, m4a_dash
  140 m4a                  │     4.13MiB 127k https │ audio only       mp4a.40.2  127k 44100Hz medium, m4a_dash
  251 webm                 │     4.11MiB 126k https │ audio only       opus       126k 48000Hz medium, webm_dash
  17  3gp   176x144      8 │     2.67MiB  82k https │ mp4v.20.3    82k mp4a.40.2    0k 22050Hz 144p
  160 mp4   192x144     25 │  1006.62KiB  30k https │ avc1.4d400b  30k video only              144p, mp4_dash
  278 webm  192x144     25 │     1.70MiB  52k https │ vp9          52k video only              144p, webm_dash
  133 mp4   320x240     25 │     2.01MiB  61k https │ avc1.4d400d  61k video only              240p, mp4_dash
  18  mp4   320x240     25 │     9.42MiB 290k https │ avc1.42001E 290k mp4a.40.2    0k 44100Hz 240p
  242 webm  320x240     25 │     2.81MiB  86k https │ vp9          86k video only              240p, webm_dash

None of these will be the format it was originally uploaded in, but I’d guess format 133 (~61 kbps) to be the closest for the video part, at 2MB. Judging by the sound of it, the original audio bit rate was probably higher than the video bit rate!

I've been looking for youtube-dl with saner and parsable -F output. Thanks for suggestion!

Without checking, I would guess it was more related to limited upload speeds: video was probably recorded at a much higher bitrate, but then uploading that...