iPhone 15 photos are not captured 24 MP size

Photos taken on the main camera at x1 are saved at 24 or 12 MP, did you check to have taken them at x1?

The setting are fine, it's the camera settings when your are shooting a photo that you need to watch, not all lenses and zooms give you the desired 24 MP, you have to make very sure you're shooting at 1x, tap the 1x button and do not zoom. It's like taking a picture with a 24mm lens. If you want to zoom in closer you'll get photos at max 12MP

The default aspect ratio on the iPhone is 4:3 since the camera sensor is 4:3. If you're changing the aspect ratio to 16:9 or square, your iPhone will use only part of the sensor, so the photo will have lower details and consequently lower megapixels. Different sizes and different lenses lead to different MP. For example I took photos with my iPhone 15 pro camera and it's as following

24MP will be at default settings for 1x 4:3 lens f/any depth>18MP 1x 16:9 f/any depth

12MP 2x 4:3 f/any depth and 9MP 2x 16:9 f/any depth

Hey, I was facing this same issue for a while and couldn't figure out what was the problem. Later i changed the aspect ratios to 4:3 so now it takes photo in 24mp. I don't when and how it changed to 16:9 and start clicking in 18mp only. I suggest you to do the same.

You're welcome :)

You have to have HEIF MAX selected when taking a picture to use the 48mp. This should be at the top right when you go to take a picture.