If you open iCloud Photo Library on your device or your photos are synced from iCloud, the case would occur.
On your device, when you open iCloud Photo Library, it will automatically upload and store your entire library into iCloud to access photos and videos from all your devices.
When using iCloud Photo Library, all the photos you see on your device are actually not the full size photos but in small size. That's the reason why the exported photos by our program are in small size.
To export the full size photos, go to Settings > iCloud > Photos > iCloud Photo Library, change the option from Optimize iPhone Storage to
Download and Keep Originals. Then, export again using our program. The exported photos will in full size.
Note: You need to re-connect your phone into our program to let your operations on phone take effect.