Google apparently has a market cap of around $1.200 Trillion based [0] largely on (for 2021) revenue of 256.7 billion U.S. dollars, of which 209.49 billion U.S. dollars came from advertising [1]. It's apparently fourth on the list of valuable companies [4]

If OpenAI takes a good chunk of Google's ad revenues then it doesn't seem that fanciful that it'll be up toward the top of market caps.

[0] https://companiesmarketcap.com/alphabet-google/marketcap/ [1] https://www.statista.com/statistics/266206/googles-annual-gl... [2] https://businessplus.ie/news/most-valuable-companies/