Market cap is most recent trade price * supply. It's not a reflection of how much money or value can be withdrawn from the market, that's more a function of market depth.

For instance if I create my own token with a supply of 1,000,000,000 and I sell 1 token to my buddy for $1, I'm, now a "billionaire." That's market cap. Where did that come from, is what you're asking. The answer is, whoever controls the most recent transaction.

However, the total market for my shitcoin is my buddy, and the total amount I can get back out is $1. That's a function of market depth.