Your iphone takes a picture in 4K, big enough to fill a cinema screen.

Most people look at the web on smaller screens.

Instagram use images 1000pixels  by 1000 pixels which is a good ball-park size for images on the web.

To reduce an image size on a mac, make a copy of your original, and open it in preview.

Select Tools>Adjust image Size…

Set the longest edge to 1000 pixels

Select OK

Save your image

Who does this?

Anyone who wants their webpage to load faster.