I think on our side we ask for the maximum size you have Opus set to, and from the screenshot it looks like something smaller is returned (or something smaller that has been scaled up).
That might be because PDF-XChange limits the size, or it might be because it is using an embedded thumbnail within the PDF files which is too small. So it could also be due to the software that creates the PDF files, rather than PDF-XChange.
It's mostly a black box to us, and not our code, so you might get a better answer form the PDF-XChange authors.