Go to the profile of the user whose profile picture you want to enlarge. You can do this by tapping the search icon at the bottom of the screen and entering the username.
To enlarge the profile picture, simply use the zoom gesture on your smartphone. Place two fingers on the screen and spread them apart to zoom in on the image. If you want to take a closer look at the image, you can enlarge it further by spreading your fingers further apart.
If you want to save the enlarged profile picture, you can take a screenshot of it. Press the power button and volume down button (on most smartphones) at the same time to take a screenshot. The image will then be saved to your gallery.