Maybe you're not familiar with how long GOP encoding works with IPB frames? If all frames were I-frames, maybe what you're thinking might work. Everything you need is in the one frame to be able to describe every single pixel in that frame. Once you start using P-frames, you have to hold on to data from the I-frame to decode the P-frame. With B-frames, you might need data from frames not yet decoded as the are bi-direction references.

Still you don't necessarily need to have dynamic memory allocations if the number of deltas you have is bounded. In some codecs I could definitely see those having a varying size depending on the amount of change going on in the scene.

I'm not a codec developer, I'm only coming at this from an outside/intuitive perspective. Generally, performance concerned parties want to minimize heap allocations, so I'm interested in this as how it applies in codec architecture. Codecs seem so complex to me, with so much inscrutable shit going on, but then heap allocations aren't optimized out? Seems like there has to be a very good reason for this.