Transferring photos to jpeg the resolution is too small

Are you asking about transferring photos from an iPhone to a Mac?

Is the system version for your Mac shown below your question still valid?

How have you been checking the resolution of the transferred photos? In Photos? In the Finder on your Mac?

This may happen, if you are just saving the embedded JPEG preview of a RAW file.

If you have been shooting RAW it will depend on the RAW support for your camera model on your system version.

For the system version given below your question the list of supported camera models is here: Digital camera RAW formats supported by iOS 14, iPadOS 14, and macOS Big Sur - Apple Support

Photos for Mac can use the RAW support directly, when you edit RAW photos on Mac, but there is a hitch, when you try to edit a RAW photo on your iPhone or iPad. Then only third party apps will actually edit the RAW file. Photos for Mac is just faking it and will edit only the embedded JPEG preview, even if your camera model is supported on your iPhone or iPad. The edited photos may then have a poor resolution, because the JPEG previews tend to be smaller than the RAW file. have you edited RAW photos on your iPhone or iPad in Photos (and not in an external editor like Photoshop) and transferred them back to your Mac? Then the resolution will be decreased.

What was the source of the photos and the original resolution?

Which transfer method did you use, among those listed in the related article?

What resolution did you expect?

What resolution did you get?