XDA Developer's review has JPG files with EXIF data for iPhone 13 Pro Max. Based on the sample photos. the sensor size of the main camera is 1/1.7" and the telephoto/ultrawide cameras have 1/3.6". The main sensor area is around 3.6x larger than the telephoto/ultrawide.
The main sensor is now larger and faster than those 1/2.3" superzoom compact cameras (PowerShot SX730 HS, Nikon P1000). It even collects more light than the Sony RX100 VII- RX100 VII min aperture of f2.8 with a 2.72x crop factor (one-inch sensor) is equivalent to a full-frame f7.6. iPhone 13 pro max aperture of f1.5 with 4.55x crop factor is equivalent to a full-frame f6.8. Compared to budget DSLR with kit lens f3.5 and Sony RX100 with f1.8, the iPhone 13 pro main sensor is one-stop slower.
Most of the ultrawide low light improvements came from the larger aperture of f1.8. Compared to the f2.4 on the iPhone 12 pro, the f1.8 offers a one-stop light boost.
The 77mm telephoto f2.8 is 2/3 stops slower than the f2.2 65mm telephoto on the 12 pro max and one-stop slower than the 52mm f2.0 telephoto on the 11 pro.