Perhaps the most realistic definition.
You could also restate it as "most people don't have lots of money"
Regardless of how much you yourself actually have at the time.
I think it's good to look at "capital" as "other people's money" (OPM).
Of course if you have enough of your own you might not really need any of OPM if you don't want to get involved.
OTOH in that case you might be one lending out some of your own "underutilized" money to others who are "interested" in OPM of some sort.
The repayment includes the interest intended to compensate for the risk & delay in repayment, so it simply means more of OPM is coming back than yours that went out.
If none of this OPM was changing hands there would still be the same amount of capital.
But the capital-ism where the system is structured so financial rewards of labor are subjugated to the financial rewards of capital itself according to its magnitude would not be as much of an actual thing.
And it's this feature that can be exploited by greed in the most disadvantageous way compared to less OPM-oriented financial foundations. Regardless of the mathematically enhanced returns to the most shrewd capitalists, the greed itself subtracts from what actually could have been overall.
As the magnitude of the money that most people don't have a lot of, becomes less significant compared to the some people who have lots of money, the unlevel playing field tends to tilt toward the vertical as a function of greed.
That's a very narrow definition of capitalism. It's not just 'stuff' that is privately held, it is also the means to generate profit and a very large chunk of the available capital. The effects of that go way beyond just 'stuff', and can make it next to impossible for someone born at the bottom of the stack to work their way up. Of course, some inevitably succeed at this and they are held up with great fanfare to prove that 'the system works' but on average, if you are born at the bottom your life will likely continue to at that level of society without much chance of upward mobility because the system is stacked against you.
And payday loans at exorbitant rates are a mechanism that keeps people down.