Why does "volo add someLibrary/iWantToUse" install the entire library source code, rather than just the consumable pieces?
Browser-based libraries come in a variety of shapes and sizes, and 'volo add' often isn't able to guess exactly how they're intended to be consumed. Fortunately, it's usually easy to create a packages.json file that tells volo this info so that other users don't run into the same road bump you did. More detailed information about volo's support for some of the libraries useful to gamers is here.
That effort has been superceded by combination of WebGameStub, which is much more tightly focused on a 2D canvas game, and the minimal-app-template, which is really intended as more a "base class" upon which other templates (including WebGameStub!) are based. Much of the work that went into the older release is still part of the WebGameStub and minimal-app-template.