GitHub is a large place with good things and not so good things. Personally, I think this user script is a must for everybody.
I've always missed being able to have some sense of the file weight but now that I navigate with it, I don't know how I put up with this huge missing feature for so long.
Anyhow, I finally decided to fix it myself. This script should work in any GreaseMonkey/TamperMonkey type of extension.
It's not flawless; sometimes it may skip a file because the DOM is what it is but nothing a quick refresh won't fix. And it doesn't happen often. Still, I'll keep improving it.
Just try it out, I guarantee you'll enjoy it.
PS: You have two viewing options for small files. You can either see > 1KB or something like 768 B, meaning bytes. Up to you!