I believe the OP is asking why does the image come in at such a low zoom ratio. When I open jpegs in floating windows they are zoomed to the largest "standard" ratio, i.e. 12.5%, 25%, 33%, 50%, 66%, 100%, that will fit the available screen area (and no larger than 100%). I was not able to duplicate the OP's issue on my W10 system with the latest version of Photoshop CC, didn't find any settings that would bear on the issue.
Richard Southworth