Hi,
It is because you have "tall" (portrait) product pictures instead of "wide".
To fix this height sizing problem, open this JS file:
www.kcmtshop.sk/templates/flexible_lightmart/js/template.js
find these lines below, and remove them completely (or comment out them):
//Make fixed height for Priduct Details page
var ProductDetailsFixheight = $('#vmFlyPageTop').height();
$('#vmFlyPageTop').css("height",ProductDetailsFixheight);
Therefore you won't have a fixed height for product details page. The height will be auto depends on how tall (portrait) product picture you have.
Regards.
Support.