Earlier quoted context omitted.
Sure, download and run the javascript, but then you can snapshot the DOM, grab the text, and discard all the rest. The HTML and js is of little practical value for the index after that point. Google's index is likely very large because they don't have any real economic incentives to keeping it small.
>... but then you can snapshot the DOM, grab the text, and discard all the rest Yes, absolutely, I didn't mean to imply otherwise. But first you have to figure out what you can discard beyond the HTML tags themselves to avoid indexing all the garbage that is on each and every page. When I tried to do this I came to the conclusion that I needed to actually render the page to find out where on the page a particular pie…
The couple of kilobytes per document is the actual storage footprint. Sure you need to massage the data, but that almost entirely CPU bound. You also need a lot of RAM for keeping the hot parts of the index.