Lightbox image size

Hi there,
I bought the commercial version.

I uploaded all images in 800x500px, but the lightbox shows only images of half that size.
Attached full-scree view shows which popup dimensions 400×250
Altering the popup width and height only gives a black border.

Screen captures available.

This concerns webpage: http://www.ruudvanuden.nl/de-beursstand/

NOTE:
I asked this question a few times through E-mail, but except one short answer, nothing happened afterwards to help me out.

https://wordpress.org/plugins/portfolio-gallery/