It's the pages. It's all the JavaScript. And especially the HTML5 stuff. The amount of code that is executed in a webpage these days is staggering. And JS isn't exactly a computationally modest language.

Of the 200kB loaded on a typical Wikipedia page, about 85kb of it is JS and CSS.

Another 45kB for a single SVG, which in complex cases is a computationally nontrivial image format.