On the one hand, EXIF apps/extensions, such as ViewExif or Metapho, are not 100% reliable when it comes to the meta information of RAW files in the iOS Photos app.
On the other hand, some third-party camera apps that save DNG/RAW files save the RAW file additionally as a JPEG or HEIC file. It is an iOS feature that these two photo versions may be represented as only one thumbnail. But these two photo vesions may also be represented as two thumbnails. Which one is the case, can be determined by the third-party camera app that saves the photos.
This case that only one thumbnail is displayed in the Photos app for the two photo versions results in some photo editing/displaying/EXIF apps getting confused and picking the wrong photo version.
So, you may have one image, which has been saved a DNG file and a JPEG file, both files being displayed as only one thumbnail in the Photos app. The RAW Power and/or your EXIF extension/app might have picked the JPEG file instead of the RAW file, which would explain the size of 3.5 MB.
If this was in fact the case, the RAW Power app would indicate that you have opened a JPEG and not a RAW file. For example, some RAW-specific sliders are not visible.
For this reason, I prefer a camera app that saves the DNG file and the HEIC/JPEG separately, being displayed as two thumbnails in the Photos app, namely, Camera+ 2.