Thumbnail behaviour for PNGs depends on file size not file dimensions. The threshold seems to be 12 kB. Anything below gets displayed full size, no hyperlink, and includes file information caption (ex "pcvhose.png (11.4 KiB)"). Anything above is displayed in reduced size, hyperlinked to the original file, and no info caption.

I think this is undesirable behavior. As has been demonstrated, this creates an inconsistent viewing experience for images with the same dimensions.
I do not believe that PNGs of identical dimensions will differ so drastically in download time, that they would need to be displayed differently. The thumbnailing algorithm should be based on dimension, not file size.

See this topic for demonstration:

https://www.phpbb.com/community/viewtopic.php?t=2629321