In article .com,
Savageduck wrote:

There are also many examples of use. For instance, there is one
"-thumbnail":

https://www.imagemagick.org/Usage/resize/#thumbnail

A whole chapter of it:

https://www.imagemagick.org/Usage/thumbnails/

...and in my opinion, all of that appears to be a royal PIA.

exactly.


And it has some information that may explain what is happening to the OP:

«Many images from digital cameras, scanning software, and some paint
programs (photoshop is notorious for this), save extra information about
the image in the form of profiles. This includes image formats such a
JPEG, PNG, TIFF and as of IM v6.2.4-1 GIF. Of course the IM specific
format, MIFF also does this. (See Image Profiles for more detailed
information).»

«These profiles can be up to 60 Kb in size, so can make a big difference
to your file size, and by default IM will preserve this profile
information. Thumbnails have no need for this data and often not even
the main image needs it.»

...but why the need for thumbnails in the first place, or have I missed
something?

still unanswered.