Since Twig is a lineart iconset, we needed to make icons specifically for various sizes so as to retain the same line thickness. You will find these icons in the respective folders for 12px, 16px, 24px, 32px & 48px. Some of these folders contain less icons than others, use the 16px one by default.
If you want to contribute some new icons, knock yourself out; just send us a black and white, SVG version of it and we'll consider adding it to the repo.
MIT license and all. Don't be a dick.