If you enlarge the image without adding extra pixels, the size of each pixel must increase. The result will be fewer pixels per inch, therefore lower resolution. The more you enlarge, the more visible the pixels become. The result is called staircasing, aliasing or good old-fashioned jaggies. Of course, the opposite occurs when you reduce the size without removing pixels.
Pixels become smaller, so the resolution increases.
Visually, this is not a problem, but the resolution could become unnecessarily high when compared with output requirements. Keeping image resolution in the correct relationship to the intended output device minimizes file sizes and ensures efficient processing and printing. When you need to resize an image, you should adapt the scanning resolution accordingly. So your adapted scanning resolution needs to be three times as high as the desired final image resolution. If you want a final resolution of 200, you need to scan at 600 ppi.