Does dpreview gallery recompress photos?
I did the test you suggested and recompression is evident, they leave unchanged the pixel size though.
Pity, because the slideshow is nice, but recompression is so strong I could notice it even with no zoom, then zooming, and even more comparing original vs full size dowload, revealed the problem.
|_MG_5100 by tim and jan|
from Welcome to the Saloon!
|The Grimm 11 year old by Ryan Gardner|
from Trick or Treat
|Heron with fish by APenza|
from A Big Year - birds