Even for big streamers, the quality goes up and down depending on how they’re moving and what the map is.
Though I looked at your stream, a little more closely this time, and IMO it doesn’t look like there’s anything wrong with the quality. If it’s 6.2 MB and not 6.2 Mb (‘B’ = 8 bits = 1 byte, ‘b’ = 1 bit = 1/8 byte), then that’s way more than enough the max is 3500 Kbps = 0.4375 MBps like you said.