Lightbox displays images at a reduced size. For comparison, XenForo 2.3.6 displays the same images significantly larger. I am unsure exactly which version introduced this regression.
Example as seen with 2.3.10. This image is 2500 × 1667.
As you can see in this test thread, the image that is clicked on first is reduced in size in the display.
Although it has the same dimensions as the others...
If you use the <> keys to scroll through everything, everything is fine!