Why are iPhone photos so big?

Resolution. A 12-megapixel photo is twelve million pixels; a 48-megapixel one is four times that. More pixels means more detail to describe, and file size rises roughly in step with pixel count.

HDR. Recent iPhones store extra brightness information so bright skies and dark shadows both survive. That gain map is extra data on top of the picture itself.

Live Photos. Unless you turn it off, every shot also records about a second and a half of video either side of the shutter. The video sits beside the still and can easily double what the shot takes up on disk.

ProRAW and ProRes. If you have deliberately turned these on, files get very large very quickly, because that is the point of them — they keep the sensor data for editing rather than a finished picture.

If the problem is an upload form or an email limit, the fix is almost always to reduce the pixel dimensions, not to fiddle with the quality slider. A 48-megapixel photo viewed on a phone screen or printed at 6×4 is showing a small fraction of its detail anyway.

Compress an image works the other way round from most tools: name the size you need — 200 KB for a form, 25 MB for an email — and it finds the highest quality that fits, rather than asking you to guess.

If a service specifically needs certain pixel dimensions, resize the image to exactly those instead.

Turning Live Photos off in the Camera app removes the attached video from new shots, and is the single biggest saving for most people.

Leave the format on High Efficiency rather than Most Compatible. HEIC files are about half the size of the JPG equivalent — the trade is that you will sometimes need to convert one to JPG before sending it.

If storage rather than sending is the problem, the photos already on the phone are the bulk of it. Converting an archive to HEIC or WebP recovers a lot of space, but do it on copies and keep the originals of anything you care about.