Reddit.com HTML.: 37.6 KB Text in that HTML: 5.9 KB
(That's just what I got from copy+pasting.) The difference at issue is loading another 37.6 KB page versus loading whatever part of that 5.9 KB is the "interesting" text (i.e. probably not the header or footer) plus its links and styles and the overhead of whatever you want to stick it in (JSON, an HTML fragment, etc). The huge CSS and JS files are not reloaded every time you go to a new page.