SVG: Scalable Vector Graphics - can be rendered at any size.
Somehow unrelated, many people showed how at low resolution vector graphics rendering alter the perceptual nature of icons compared to bitmaps (crafted by human eyes for human eyes). So I believed the author made vector shapes, with the goal of very small "ascii art" flavor baked in.
Yes. You can't beat PNG in low resolution, because it's hard to make SVG pixel-prefect on all resolutions. But on modern devices, with high DPI - it's not a problem. And you will get a high size reduction (but with some runtime cost).