S&P 500 is primarily a ranking by market cap. It’s a measure of how much the market values a stock. I can see it being a component of size (how “big”). There’s no doubt that the big tech companies are colossal entities.

The market cap of a company is largely based on a bet of the “size” of a company in the future.

Tesla is the most “valuable” US auto maker, by market capitalization. This illustrates that market cap is inherently related to predictions which have a large speculative component, which can be influenced by pure market phenomena.