systematic reduction in quality of whatever I upload, whatever the resolution

I would like to know how youtube works:
It can feature a variety of formats for a single video, as shown by youtube-dl "blablablabla" -F, including webm and mkv.
The files I upload aren't of the best qualities sometimes... vintage recordings.
So it is paramount that the files are uploaded as is, nor processed or whatever.
But even with low size (below 200 Mo) .mp4 files it ALWAYS, ALWAYS process them down to something lower in size, or even LESS THAN A TENTH the original size, when the original is around 1 Go.
Can I stop this from happening ?

whatever the format or encoding I use I export the video in it's always reduced in quality. Nothing to do with resolution either.