Is lightbox slideshow limited ?

Hello ,
I upload images in this 1500×1000 in my galleries. I have well define the maximum size of 1500×1500 in setting=>image=>image preview size.

So, when I’m on the gallery and I lunch the lightbox slideshow (black background with arrows prev/next), the image do not appeared in 1500×1500 on the screen !
If i copy it to my computer, I can see the image have the good size (1500×1000).
But, the image do not show it self at the good size in the lighboxe slideshow ! It appeard smaller 🙁 ….
So I wonder what does limit the image to appaerd with it real size ?
Do you know how to do that ?

Alex