OK, so on a newly created image, it defaults to Points, and creates tiny text. Switching to Fixed does indeed make the text larger.
If I load an image, it defaults to Points, and creates larger text. Switching to Fixed has no apparent effect on text size.
What is interesting, is if I create the image as "pixels/inch", it defaults to 96dpi -- and text appears as the correct size regardless of what the Metric is set to.
If I create the image as "pixels/cm", it gives 37.8, which (37.8 * 2.54 = 96.012) is close to 96dpi, then the Metric does have an effect.
So, I think there is a bug in there somewhere!