I would more simply define that as "wealth inequality" rather than capitalism (or more broadly, power inequality) and perhaps go on to say that the real problem is that, while you can't realistically prevent/remove all inquality, most systems do a poor job of preventing the people with more money/power from using that to consistently increase their own share.
No need to do that, Capitalism—just like feudalism and mercantilism before it—is about how power is distributed. Both feudalism and mercantilism also had extreme wealth (and power) inequality (feudalism arguably had more), as did communism for that matter.
What makes capitalism different is that the power is distributed along capital (as opposed to handpicked by the king under feudalism; or embedded in government monopoly under mercantilism).
In other words, in capitalism, the owners of capital (or the owners of the means of production; i.e the rich; the aristocracy; etc.) are the ones who get to dictate the living conditions of the rest of society. The rulers (be it democratically elected government; an absolute monarch; a military dictatorship) will legislate in order to maintain the interest of the owners of capital. The police (or military) will fight for the interest of the owners of capital, and will suppress any resistance against the interest of the rich, etc.