While waiting for a colleague to take this ticket I took a quick look at your site.
From the front end I can see why the presentation of the images is different.
As you can see in my screenshot, for the img elements, with some of the images (the problem images) the sizes attribute includes "auto", but for the images which look correct they do not.
I'm not sure if that is enough to help you identify what the source of the problem may be.
I can't tell exactly how you have made this page (I see that it is a category archive).
If you need more help then I think we would need to be able to access your site to take a look at how the page is made, so let me set a private reply now in case you want to share credentials with us to examine your site.