actually it paints per polygon and not per pixel. if the density of your polygons is lower than the density of pixels on your uvw map, then that's why it will come out pixelated. for example, if you're only subdivided to say 400,000 polygons, your polygon density might be 1 vertex per 8 surrounding pixels at 2048*2048…
Zbrush isn't the best for poly painting as i've found out. Even at high poly counts it paints per pixel not polygon. The texture resolution is heavily dependent on how much you are zoomed in or out. I've tried 3D coat for poly painting, and it is amazing. You can pain on a low poly mesh in real time just as you would…