Leaker Sonny Dickson has shown iPhone 13 dummies of all four models, alongside case renders and video of a case (below).
Both dummy models and case renders show the same small changes over the iPhone 12 …
We can see a smaller notch in the case renders, and both renders and dummies show a somewhat different camera module placement on the standard model.
Additionally, Dickson suggests that the iPhone 13 Pro Max dummy is slightly larger than the iPhone 12 Pro Max.
It’s not surprising that both dummy models and cases/case renders are consistent: Both are usually based on the same leaked CAD drawings. These drawings can, of course, be genuine or faked, so while they can provide an early look at what to expect, they are far from reliable.