On top of what other people have said about parallelism overheads, you normally need more data to train a bigger network and the training time is roughly proportional to network size * training data.

IIRC OpenAI used a million times more data to train GPT3 than karpathy used in this video, so a naive estimate would be that it would take about 20 billion times more compute. This is could be a significant overestimate since Karpathy probably used each bit of the training set more times than openAI did.