What controls the dimensions of an IPhone 12 captured image?

I am comparing the images my IPhone 12 takes with those produced by other phones.

The pixel dimensions of the images my IPhone takes are 1536x2048, whereas the pixel dimensions of images taken by a OnePlus6 are 3456x4608 (which is closer to what I was expecting from the IPhone).

So my question is - What is controlling the size (in pixels) of the images that my IPHone is taking? and why isnt it using all the pixels that it supposedly has, and how can I convince the phone to take bigger (in terms of pixels) images?

(obviously this is not a question about Mbytes on the disk)

Solution 1:

I was downloading the images from icloud.com to my windows PC and it was there that the problem occurred. the link you supplied gave the answer - a "long click" on the download button gives a pop up menu from which i could then choose "original size", so now I am getting 3023x4032 images. Still not quite as good as the oneplus, but in the same region.