Pixel resolution reduction when editing photos

Hi all, I've encountered a problem with my iPhone reducing the resolution of photos edited by third-party applications and I'd appreciate your advice for a solution.

If I take a photo using my iPhone, then use the Google Photos app to edit the photo (not cropping), the edited JPEG that ends up back in my iPhone's 'All Photos' folder is of reduced pixel resolution. So if I email, message, or publish the edited picture using another iPhone app, I get a the photo with a reduced resolution.

The resolution drop is significant. The original photos are 4032 x 3024, the reduced resolutions are 1792 x 1344, and the quality drop is noticeable even to my untrained eye.

This is not a problem with the Google Photos app per say. If I use the Google app to directly export the edited photo then the resolution remains as-shot. Likewise the photo which gets uploaded to the Google Photos 'cloud' is fine. The resolution reduction is occurring when the edited photo is passed back to the iPhone photo library and seemingly a second, lower resolution JPEG is produced. The JPEG uploaded to iCloud has the reduced resolution (though the original, unedited HEIC file remains available too). The same thing is happening on both my iPhone and iPad.

Do you know what's causing this? Is there a way to force my iPhone to retain the full-sized, edited photo?

Below is an example which demonstrates this problem. The photo was take using my iPhone; I used Google Photos to reduce the colour saturation, then compared the two JPGs uploaded by Google and iCloud.

Version Uploaded by Google Photos (4032 x 3024)

Version Uploaded by iCloud (1792 x 1344)

Many thanks,

Mark