Ken Hall writes:

I understand that upsampling can result in a poor quality image, but I
have always been under the impression that downsampling does not
reduce the image quality.

You have to be more precise about what "image quality" means to,
particularly the size of the image when you are looking at it.

Upsampling results in *the same* quality image when viewed with the
images all the same size, or it can be better if it reduces staircasing
along edges. Downsampling reduces the image quality at the same size,
because there are fewer pixels and less fine detail.

On the other hand, if you're talking about looking at a small area of
the image at 100% scale, then the comments above make more sense.
Upsampling spreads the same information over more pixels, so the image
looks less sharp at 100%. Downsampling usually gives an image that
looks sharper at 100%, because features are smaller - but there is less
information.

The view at 100% is not what matters - it's the view of the full image
that's important.

Dave