That is a bet and not a metric of company size. Some people bet a lot on small companies, doesn't make them large.

Investors look at how much money is already invested in a company in deciding whether to invest. I.e., investors pay close attention to market cap.

If Google's market cap were $25 trillion, practically nobody would buy Google stock (and practically everyone who already held the stock would immediately sell) because most investors do not believe that Google can ever pay enough dividends or buy back enough stock to justify such a high valuation.

A company's market cap is a collective estimate of how much money the company will to return to investors in the future. When the company is publicly-traded in an open informational regime such as the US, this collective estimate is usually quite "accurate" in the sense that it is very difficult for any single analyst or single team of analysts to improve on the estimate.

An investor can make a big bet on a small company, yes, but the market cap of a company is more than just an indication of how much money has been bet on the company: it also mean that every investor (big or small) who still holds the stock believes that the expected amount of money that company will return to shareholders exceeds the market cap: if there were a holder of Google stock that did not believe that, he would convert the shares into treasury bills or cash in the bank.