The answer to the question is that the images I was looking at were svg files (they were not in a ltximg subdirectory, but in ~/.emacs.d/.local/cache/org-latex, maybe something to do with Doom). I learned from this question that the blurriness is a known issue in Emacs.
You can give to an image an atrribute of :width to specify its width. For example:
#+ATTR_LATEX: :width 0.3\linewidth
would do the job.
But there is the additional complication here that the image file is generated by a source block. The easiest solution IME is as follows:
Give the source block a name.
Generate the results with C-c C-c -- ...