The pixel dimensions of the saved image should not be (and probably isn’t) changing. If I open a 1704pc by 2272px JPG in JPEG to RAW AI, then save as a DNG, the DNG image is still 1704px by 2272px.
I suspect you are reading the properties of the DNG file using Windows File Explorer properties->details. Unfortunately, for DNG that doesn’t show the full size of the image, even for DNG’s produced by Adobe Digital Negative Converter. If I remember correctly it is showing the size of a preview image embedded in the DNG file.
I suggest you check the RAW image size using software that supports RAW developing. (Photoshop, Affinity Photo, ACDsee etc).