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
Save your image
Who does this?
Anyone who wants their webpage to load faster.