You can also use Tinkercad (tinkercad.com) to import an .stl and export an SVG.
Here are a couple of tutorials:
Actually cairo is not rendering the gradients at 16-bit bit per channel depth. There is a open issue about this. See:
RGBA16 png export: no difference between RGB-8 and RGB-16
Now as this is still just speculation i had a look in cairo source. Yes this is the case. Also verified results in mathematica and photoshop.
So all you get is a 8 bit image in a 16 ...
The histogram of your image looks like this:
The histogram of the SVG loaded directly in Gimp or loaded in Inkscape, and exported to PNG and loaded in Gimp are similar. The important features are the gaps and the spikes. When you stretch contrast (and to obtain the image you show above from your SVG you increase contrast a lot) you are in effect widening ...