It has to load all the blocks on the screen, I think even the air counts as chunks. Thats why the game is just really demanding onto the computer
blegh this ended up being a lot longer than intended, sorryyyyyyy.
The air counts as chunks in terms of data but they obviously have 0 toll when rendering. Even visible blocks will only ever need to render a max of 6 polygons each (2 poly per face, only 3 faces can be visible at a time), and will almost always only need to render 2 or 4 polys. So even if you look out into the distance, that is what, maybe 1000 or so polys being rendered, possibly more if it's a *really* complex landscape. Most of the time you're just mining or whatever, i.e. looking at a wall, you'd be rendering maybe 24 polys or so. Let's say you're out looking around inside a house you built or the mine is fairly large, still only maybe 100 polys being rendered.
Seriously in terms of rendering Minecraft is not complex at all. >.>
Compared to Runescape which I imagine has what, 70-100+ poly per character, plus landscape (let's say 100poly visible at a time), random objects (trees (50poly), fences (20poly), etc.), not to mention buildings. Even if you have a completely empty landscape with one player in it, more polys are being rendered per frame than in a typical Minecraft setting. The main thing Runescape has going for it is a much lesser draw distance, but you can limit that in Minecraft anyway.
Sorry if I sound forceful at all, it's not the intention, just trying to explain my point of view.