The idea is that if you have more tokens, you will reap more reward in PoS. This is because you receive around 5% return on your stake, so if you set up multiple validators you will earn more per year.

Note that PoW works exactly like this too, except far worse due to economies of scale. Purchasing mining facilities in bulk will give you a much better deal than buying rigs as a home miner. Whereas in PoS, both the ultra-whale (1000 ETH) and mini-whale (32 ETH) are earning the same exact % return on their investment. The average user delegating in a staking pool is also earning fairly similar return to all validators (with some % of their return paid to validators for their services).

I do not think the claim that PoS is more centralizing or more “rich get richer” than PoW is necessarily true.