Why do you think ChatGPT doesn't use a quant? GPT-OSS, which OpenAI released as open weights, uses a 4 bit quant, which is in some ways a sweet spot, it loses a small amount of performance in exchange for a very large reduction in memory usage compared to something like fp16. I think it's perfectly reasonable to expect that ChatGPT also uses the same technique, but we don't know because their SOTA models aren't open.
