Sure, individual situations vary but you can average across small stakeholders and conclude that they lose out as a group. Unless of course the group is self-selected to include only those with the necessary resources, but this is still going to be wealth-concentrating in general.

Another factor which others were trying to convey is the risk-adjusted return per stake is lower for smaller stake even if pure expected value per stake is the same.

resource requirements are pretty low for most PoS systems

I think this basically assumes the system is a failure, no (or at least, if successful only in a very niche way)? For something with "high" usage (say as high as current Ethereum or potentially much higher), the resource requirements will not be low.