I think the 500 tokens comes from the observation that an idea is encapsulated in 1 to 3 paragraphs, and 500 tokens is up to 3 average paragraphs. The 1000 token completion just makes sense since it gives the model some room to breath and contain its output ideas to less than 5-10 ideas max. You probably don’t want the model to create too many more output ideas than input ideas, otherwise it can start to drift. At least that is my theory.