When using the masonry grid on the start page, the featured images are resized by width / height, but not with an own thumbnail of the original.
So the images get resized to a third of their dimensions, but their file size stays big.
However, we don’t currently have different image size options for featured posts. This means you want to set the image width to wide enough for your featured post, and then they will resize down when they’re no longer featured/in columns.