Galery looks strange, Lightbox much too big

for that small images below – there is in relation to its own dimension a big padding (7px for images that are approx. 30px)
so set the padding to a smaller value:

#top div .avia-gallery .avia-gallery-big,
#top div .avia-gallery img {
  padding: 2px;
}

here the big image above is included to that new padding – so that the large image is aligned with the block of small images.

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I always set my lightbox so that the background does not scroll.
There is now a filter for this which you can set in the child-theme functions.php.

add_filter( 'avf_default_lightbox_no_scroll', '__return_true' );

Apart from that, I don’t see the reason why these extreme heights are set for mfp-bg and mfp-wrap. The usual values are 1600 to a little over 2000px, why is it twice that in your installation? …