One other remote possibility. There is an ASCOM command to ask the camera for the pixel size. I bought a Starlight Xpress Superstar and it reported the wrong pixel size. PHD2 then got confused, as it read the pixel size from the camera and got the scaling wrong. It was fixed in 24 hours with a firmware patch for the camera.
Might be worth checking that the camera is reporting the correct size. You can use PHD2 to do that by connecting to it temporarily and read it in the setup screens, or do a short script - querying PixelSizeX and PixelSizeY properties.