[IggleSniggle]

It’s not the entire HTML document, it’s piecemeal. As for sizing equivalent data in HTML vs js, I remember reading an investigation that XML and JSON are generally closer than you would expect and that for some data structures, XML is terser.

[deergomoo]

It's not XML though, it's HTML. HTML with additional nodes for layout, class names, etc.

If you're not invalidating the cached code multiple times per day (which, in fairness, a lot of the stuff from Google, Facebook, etc probably are), an SPA should use less data over time than a comparable server rendered app. The initial outlay is bigger, but once it's cached you only need the raw data as you navigate around.