Facebook no longer allows you to view the original, full-resolution profile picture of someone else if they have privacy settings enabled. In the past, you could right-click and open an image in a new tab. Now, Facebook serves images in a compressed, low-resolution format (usually 320x320 or 720x720 pixels) to prevent scraping.
If you need a clean, large version of your own picture, use the "Download Your Information" tool. For others, respect their privacy—Facebook intentionally blocks full-size viewing. facebook full size profile picture viewer