The second row of the main page states this: "The first concern of toy is to bring the thinnest and simplest stack of technology for making games directly from C++, instead of focusing on a monolithic editor, or insulating the user from the core systems."

So I think this is "thin and wide", instead of "thin and small". Having all the components you need and having them directly accessible counts as thin in my opinion.

It's thin in the way that all the goodies are in separate modules that you don't have to use, and also in the way that it still runs in the browser with all these goodies.

But it's also not so thin, you are right :)

Look at the example code further down: the engine isn't a heavy framework with tons of abstractions between the big systems and the game code. You get to directly access and drive all the individual systems yourself.

The most common "big" game engines don't allow that. There you get a big, quite opaque runtime that somehow will do rendering, sound, physics and networking and occasionally call individual fragments of your game logic in between.