Sorry for the delay – I haven’t had time to set up a fresh install until today. I’ve set it up with the same images, titles and excerpts as the live site and only GP (+ GP Child Site), GP Premium, WPSP and WPSP Pro (and also Classic Editor) installed. As expected/desired in Firefox with the same image size issues occurring in Chrome and Safari:
I can provide a login if you need one.
On the original Jaijiel test site (/testsites/knock2020/) I have also tried stripping out all CSS, and disabling all plugins apart from GP Premium and WPSP/Pro. Same problem with Chrome and Safari.