If a stub template includes an image size declaration that includes "px" then that image is shown at native resolution in the editor.
This was observed with [[Template:Cornwall-stub]] which originally had the code:
|image = Flag of Cornwall.svg|
|pix = 30px|
|subject = [[Cornwall]]|
|category = Cornwall stubs|
|name = Template:Cornwall-stub|
When the "pix" parameter was changed to just "30" [https://en.wikipedia.org/w/index.php?title=Template:Cornwall-stub&diff=564819917&oldid=547286019] as was used by e.g. [[Template:Nigeria-stub]] the image size was displayed correctly. The icon has been rendering at correct size when rendered for reading, presumably since [https://en.wikipedia.org/w/index.php?title=Template:Cornwall-stub&diff=next&oldid=235904748 2009].
Interestingly it renders as intended in VE when the template is initially placed, but does on every subsequent time it's opened in VE.
Feel free to use [[User:Thryduulf/stubtemplate]] for testing (I'll leave it up at least until this bug is closed) as I have altered the actual Cornwall-stub template.