I understand how valuations work but I guess where I'm confused is that people on HN seem to conflate
market value with
size or even revenue.
After some googling it seems that my intuition about the relevance of stock prices is mostly right:
https://eu.usatoday.com/story/money/columnist/krantz/2012/10...
> If the stock price falls, these investors lose money, not the company.
The stock price is entirely speculative and detached from the company's actual performance. At best it's informed by a perception of the company's performance and an expectation of how the stock price might change in the future in reaction to the company's future performance.
While shares will be worthless if the company goes bankrupt, the company won't be directly affected if the stock market plummets -- except in situations where (additional) stock can be used as a currency in lieu of actual cash, like buying out competitors.
So to answer my own question: market cap (but mostly share price really) is only a measure of company size in so far as it indicates how much money the company could generate by selling additional shares. It doesn't provide any indication of how well the company is doing financially, how many employees it has, how much market share it serves or any other measure of size BUT generally people are willing to pay more for shares of companies that are likely to grow or at least outperform their competitors in the short term.
EDIT: In other words, yes, an overhyped two man operation running at a financial loss could end up with a massive market cap but in practice it's unlikely to happen because hype rarely works that well.