Hi there, let me check if I understand your trouble correctly, you mentioned that you have created a new folder in the Photos program and then dragged photos from the SD card to that folder, only to find that all the files got compressed, is that right? Actually, you'd better import photos from your SD card instead of directly dragging them because dragging doesn't import the original image file and would compress the photo quality.
Just connect your SD card to your device > launch the Photos program > go to View and select Show Sidebar > locate your SD card > select the SD card and import your photos.