Why your camera shoots HEIC by default
Your phone switched to HEIC years ago, and the main reason is storage. Modern cameras take bigger, more detailed photos than older phones did. Using fewer bytes per photo means your phone and cloud storage take longer to fill up.
The storage math
A modern phone camera makes very large images. If every photo were a JPG, a full library would fill the phone, and Apple's cloud storage, much faster.
HEIC cuts each file to roughly half the size at the same quality. That means about twice as many photos fit in the same space.
More than a smaller JPG
HEIC is a container, not only a new file extension. It can hold the main compressed photo plus thumbnails, metadata, extra images, transparency or depth data. Apple uses this for camera features, and the library still shows it as one photo.
A JPG export usually flattens all of that into one plain image that opens anywhere. You keep the visible photo. You may lose depth data, edit history or other extra information. Keep the original if you want those options later. A copy can't bring them back.
Quality didn't drop
HEVC compression is much more efficient than JPG compression. So the smaller files don't look worse. You get quality as good as or better than JPG at about half the size.
The only cost is compatibility. That is why converting to JPG when you need it still matters.
Where HEIC still fails
Recent Apple devices handle HEIC well. Other modern operating systems have gotten better too. But web upload forms, older Windows installs, office workflows and print kiosks can still reject it. Sometimes the upload works, and then the preview or a later automatic step fails.
Because support is uneven, check the list of formats the receiving site accepts. Whether the photo opens on your own device tells you little. For anything with a deadline, JPG is usually the safe format to send.
What to do about it
Leave your camera on HEIC if you care about storage and Apple features. Convert copies to JPG whenever the place you send them needs wide support. In the iPhone camera settings, you can pick Most Compatible to shoot JPG instead. That can make sense if you always work with older software, but your photos will take more space.
Make share copies from the original at a quality high enough for where they're going. Check the result and never overwrite the source. For public sharing, check whether the copy still holds location or device details you'd rather keep private.
A practical library setup
Treat HEIC as the camera original you keep, and JPG as the format you send. Keep originals in a backed-up photo library. Put temporary exports in a separate folder. Name each batch so you can tell which files were resized or had metadata removed.
This way you don't have to choose between small files and compatibility for the whole library. You keep the smaller, richer originals and make JPG copies only for the people and systems that need them.
Test restoring from your backups, not only making them. A photo library that lives on one phone or one synced account can still be lost through deletion, account problems or file damage. Keep a separate copy of your originals before any bulk format change.