For an app like medium, client side react with that many dependencies is perhaps not the right approach in the first place?
Server Side Rendering(react or w/o react) would be a better way to do serve their content and have the added benefit support users who have no js support (noScript , terminal based browsers, screen readers? etc)
Obligatory: https://idlewords.com/talks/website_obesity.htm
(Given your numbers and the ones in the talk, it would appear medium pages have doubled in size since the talk was given).
But yeah, I think folks reach for heavyweight frameworks too quickly. You shouldn't be using React for a blog.
I just checked one of my longer articles and my website loads 11678 bytes including the article and 23.39kb, including CSS, custom fonts and and images.
I would think that 0.5mb should be more than enough.
Is it heavy for such a simple blog? Yes absolutely. Is it heavy for a modern React app? Not really. I think the crux of it is that we often rely too much on heavy frameworks for React even for simple things. It's definitely an antipattern in development today.