I've shown the 3 different ways in which images can be attached to posts.
The first one is a 300pixel wide thumbnail, which will then load the full image in the lightbox overlay
The second one is the full image, which is then scaled back down to the width of the message box (but the full image is still loaded), which you can see by looking at the page source:
and the third one is where images are attached, but not inserted in-line, so are appended as small images to the end of the message.
It's the posts where the full size images are inserted that are causing the huge page loading size.
It's down to
@hook and
@Leben how large the images should be, as it's their site, but there are things which can be done such as running additional optimization against the attachments to reduce the file size, but keeping "most" of the quality of the images (but required an addon to the software, and would need to be ran over 1.5million previously attached images).