The Figure has a white background color by default, so set :transparent on that. Cairomakie can output transparent pngs, for GLMakie I once wrote a workaround (its buffer is only RGB for technical reasons) but I’d have to look it up first.
Also, with CairoMakie (and soon with GLMakie as well) you can use px_per_unit = 2 or higher when saving to increase the resolution.