The size of images in epub output ("make epub"), isn't sufficient, and it makes it very difficult to understand the corresponding diagrams. At least this happens on my KOBO e-reader.
A workaround is to add ":width: 100%" to each image insertion, and this doesn't seem to disturb the HTML output which already looks correct. I'll submit a patch for this.
A patch has just been sent to address this: https://lists.yoctoproject.org/g/docs/message/2906
Fixed in https://git.yoctoproject.org/yocto-docs/commit/?id=848ba7bd8984cc3f4bf6b818259865011cde0476