@eireen if you paint in the Image Editor, the brush is in pixels of the image, not of the screen, so it will react as you expect.
In the 3D Viewport, you are painting on an object and that object is not made of pixels, only the screen you are watching it on is made of pixels.
In the future, with Sculpt Vertex Colors, you will be able to get this brush behavior in the Viewport…
