There are publishing settings for HTML5 (at least in 2015 FM) that allows you to control a "preferred width" of the images. As well as an option to "Generate Images in Distiller". I am not sure if they have any control over pdfs, but it did resolve my similar issue in the past where images in my HTML5 export were noticeably smaller in dimension and had an inline styling of width="80%" applied.
Does your pdf image/content within your anchored frame get exported to HTML as an image format .jpg .png, or is it still a .pdf?