ProRes offers high color fidelity and image quality with relatively low storage overhead, an ideal combination for mobile filmmaking. The codec also benefits from fairly wide adoption in existing post-production workflows and can be exported to editors like Final Cut Pro.
According to Apple’s iPhone 13 Pro specifications, ProRes in 4K at 30fps is limited to 256GB, 512GB and 1TB models. The 128GB variant can shoot ProRes at 30fps, but at a significantly reduced resolution of 1080p.
Apple does not explain the limitation, but it can be assumed that the company felt 128GB of storage was not enough for the large files 4K filming creates.