Low light requires high ISO, basically turning the video amplifier up to max, and this causes video noise. That shows up as random grain-like effects in the images, with pixels randomly brighter or darker than their neighbours.
And randomness is the enemy of compression; much of image compression relies on most pixels being the same as their neighbours, and video compression relies on one frame being largely the same as the preceding and following frames. The very randomness of noise means it takes more bytes to encode in compressed formats.
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