Problem with big thumbnails in masonry

Hi,

I’m using an masonry gallery to show posts with images. Because I would like the images to shot at the original ratio, I’ve set it to set your own format and use no-scaling. But, the no-scaling causes large thumbnails as 800kb. With 100 posts to show, this is becoming a big page to load. See the private content field for a link to this page.
Can you recomment me another way to achieve this layout? Or another setting for the post-images?

Regards,
Erwin