If you have enabled iCloud Photo Library on iPhone or iPad or your photos are synced from iCloud, the case would occur.
As iCloud Photo Library is enabled, all your photos on iOS device will be uploaded to iCloud so that you can not access them directly. The pictures would be downloaded again when you tap it to view. Otherwise, the photos are not in full size since they are stored with small size for optimizing the iPhone storage.
It is also available to export the original photos on iPhone/iPad. Please go to Settings > Photos > iCloud Photo Library, select "Download and Keep Originals" option. Next, connect your device to PC again and you will find that the photos can be previewed and exported in full size via our DoTrans software.