How did we get such a huge camera upgrade to fit?

By thinking diagonally

iPhone 13. The most advanced dual-camera system ever on iPhone. Lightning-fast A15 Bionic chip. A big leap in battery life. Durable design. Superfast 5G. And a brighter Super Retina XDR display.

Legal.(1) Data plan required. 5G is available in selected markets and through selected network providers. Speeds vary based on site conditions and network. For details on 5G support, contact your provider and see apple.com/uk/iphone/cellular.(2) The display has rounded corners that follow a beautiful curved design, and these corners are within a standard rectangle. When measured as a standard rectangular shape, the screen is 6.06 inches diagonally. Actual viewable area is less.(3) Battery life varies by use and configuration. See apple.com/uk/batteries for more information.(4) iPhone 13 is spill, splash and dust resistant and was tested under controlled laboratory conditions witha rating of IP68 under IEC standard 60529. Spill, splash and dust resistance are not permanent conditions. Resistance might decrease as a result of normal wear. Do not attempt to charge a wet iPhone; refer to the user guide for cleaning and drying instructions. Liquid damage not covered under warranty.(5) Some features may not be available for all countries or all areas. (6) Accessories sold separately.