The first image of any gallery is showed with wrong size

Really nice plugin, but i have a little issue…

The first image of every image gallery is showed smaller than all the other image.
I tested in this way:
I got 6 image with the same dimension (1000*562) and same dpi (96), no description, the thumbnail are ok, all the same; when i click to enlarge, a lightbox is showed but the first lightbox of the first image (and the scaled image too) is smaller than the other 5 images and lightboxs.
From view image info of firefox i got this data:

• The first image of the gallery is in the lightbox is 1,000px × 562px (scaled to 730px × 410px)
• All the other image are 1,000px × 562px (scaled to 950px × 534px)

The first image is scaled in a different size.

Do you have a suggestion to solve this?

http://wordpress.org/plugins/simple-lightbox/