Peter, I'm guessing you are using either Illustrator's auto trace or Corel Draw.
Fun, isn't it?
Vector is great because instead of pixels, lines and shapes are based on a mathematical sequence based on a series of points. Therefore, when scaled, either larger or smaller, the quality stays the same because it will reduce or enlarge proportionally.
In raster images, you can subtract pixels but you cannot add them back in once reduced.