The standard size picture the iPhone gives you is 2592 by 1936 pixels. The resolution of anything you are looking at on the screen or printed on paper is up to the software producing the visible output. You can choose to print at any resolution you like - 100dpi, 200, dpi, 300, 600 whatever. The number of pixels isn't changing, only the size of them, and hence the size of the picture you see.
The "Preview" software is just that - for looking at a preview of what you have. As a standard screen resolution is 72dpi, that's what it happens to use. Any greater resolution would be wasted, being beyond the display capability of the monitor.