Hello Lemonhard,
Thanks for using our app and i am truly sorry for the late reply. This is Dorothy and i hope to be able to help you.
In normal case, AnyTrans will not change the file size and quality of photos. But if you photos are stored in iCloud instead of iPhone, the photos are thumbnails in your iPhone. Thus, the photos exported will reduce the file size and quality. It will be appreciated for your generous forgiveness and kind understanding.
Would you please follow the troubleshooting steps below to solve this issue?
To fix this issue, please refer to the following suggestions.
1. Please check whether all photos are viewable on your iOS device.
2. If problem persists, please connect your iPhone to Wi-Fi. Then, please go Settings > iCloud. Please check whether you have turned iCloud photo library and choose to optimize iPhone Storage. If so, it would cause that your photos are uploaded to iCloud and do not exist in your device temporarily.
4. Then, to solve this issue, please choose to download and keep Originals. After downloading photos to your device successful, you can transfer them normally.
Please kindly tell me whether it works for you or not.
Thank you so much for your generous support and kind feedback in advance.
Best regards,
Dorothy