Thanks for the link! It looks like your HTML document was generated as “self contained” (all the scripts and images are encoded right into the document, rather than linking to external files).
Being self-contained is what makes the HTML file large (for HTML), which is why GitHub is only offering a raw view. However, even if it were smaller, GitHub would only be showing you a highlighted source view, not the rendered view. As I mentioned above, GitHub doesn’t let you view rendered HTML outside of GitHub Pages.
But nothing seems to be wrong with that version of the file — I just downloaded it, and it opens and displays as expected on my machine (I can see all the plots, and it has the usual formatting).
Can you describe in more detail the steps you took when you had trouble viewing a copy of the HTML file in your browser?
If you want people who are browsing your repo on GitHub to be able to read a rendered version of your report, the github_document output format is probably going to be a better bet than HTML. And, of course, you can use R Markdown to knit the same Rmd file to both formats, so that one is available for easy download and the other is there for easy online viewing.