Hello, UDiPhoneUser!
Thank you for using the Apple Support Communities!
I see from your post that when you are trying to copy pictures from your iPhone manually from an internal folder, but you're only able to see smaller resolution pictures.
Do you use iCloud Photos?
As noted in the article, Manage your photo and video storage:
Your photos and videos are stored on your device in their original, high-resolution version. This means that they use a lot of space on your iPhone, iPad, and iPod touch. With iCloud Photo Library, you can make more space available on your device and have your entire collection, everywhere you go.
Get started with iCloud Photo Library:
- Tap Settings > [your name] > iCloud > Photos. If you're using iOS 10.2 or earlier, tap Settings > iCloud > Photos.
- Turn on iCloud Photo Library.
- Select Optimize iPhone Storage to make more space on your device.
With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. And as long as you have enough space in iCloud, you can store as many photos and videos as you want."
Since you mentioned that you are manually accessing your iPhone's internal folders for pictures. To make sure you aren't potentially downloading thumbnail preview of your pictures, I recommend that you follow these steps from the article, Transfer photos and videos from your iPhone, iPad, or iPod touch:
"You can import photos to your PC by connecting your device to your computer and using Windows Photos app:
- Make sure that you have the latest version of iTunes on your PC. Importing photos to your PC requires iTunes 12.5.1 or later.
- Connect your iPhone, iPad, or iPod touch to your PC with a USB cable.
- You might need to unlock your iOS device using your passcode.
- You might also see a prompt on the iOS device asking you to Trust This Computer. Tap Trust or Allow to continue.
Then, follow the steps in these Microsoft knowledge base articles to learn how to import photos to Windows Photos app:
When you import videos from your iOS device to your PC, some might be rotated incorrectly in Windows Photos app. You can add these videos to iTunes to play them in the correct orientation.
If you have iCloud Photos turned on, you need to download the original, full resolution versions of your photos to your iPhone before you import to your PC. Find out how.
Photos and videos that you sync from your computer to your iOS device using iTunes can't be imported back to your computer.
Learn what to do if you can't import photos from your iPhone, iPad, or iPod touch to your computer."
I hope this information helps you!
Cheers.