How to server scaled images on masonry grid?

hey, so my site is: https://woodwaker.com/

I've used the Broadcast demo site with GP Premium and modified it a lot.

I've finished doing the layout for my site and encountered 1 problem with the category grid.

I'm using 1200px width images for featured images which is reasonable considering retina displays. I usually use 2048px for featured images on other sites. The problem is these images aren't displayed scaled on the category page and the client has to download 10 big images when entering the website. If I have 50 posts in a category this will become a big problem.

Any ideas on how to fix this? I see there are srcset versions in the code but I think the full version is downloaded anyway? Or is GTmetrix wrong?