For some reason, images are resized to a width of 180px in the post summaries on a list page. To enable full-sized images, the following addition to assets/main/scss/_custom.scss fixes this.

 1.posts .post img {
 2    float: left;
 3    width: 95%;
 4    height: auto;
 5    padding-bottom: 1rem;
 6    max-height: unset;
 7    max-width: 700px;
 8} p {

Notes: I don’t exactly know why this fix works. Noticed in firefox dev console that the image width was being set to 180px. Setting width to a %age and a max-width seems to fix it. Adding the padding-bottom to get some separation between image and following text. The .post-summary p part is needed because without it, the text will float to the right of the image and not start from below the image. All of this wouldn’t be needed if the theme did not already include css for .posts .post img that set the image width to 180px and float: left, which we are overriding here with width:auto, and resetting the float:left with clear:both.