It was just a cheap dig, but try combining this page https://github.com/ValveSoftware/Proton/issues/2907 with this extension https://github.com/findepi/user.js/blob/master/github-expand... to expand all of the missing comments. You'll get a lot of xfer for what is effectively just some plaintext.
A good chunk of the data transferred in that issue for me are images, not plaintext. Lots of screenshots of the videogame.
I'm using Safari 14.0. Do you have any measurements (maybe on another browser) that show GitHub serving particularly large pages?