Thus, large account holders receive some amplification, but the marginal benefits of increasing account size shrink as the account size grows.

I think it makes more sense to think if it as scaling down rather than amplifying (the amplification is generally less than 1.0).

The goal here was to discourage large stakeholders from voting with multiple accounts, since vote-splitting for large accounts would be deboosted. This was intended to make it easier to find and counter the worst forms of abuse.

This was the stated goal, but 1) it wasn't at all clear that that kind of abuse was happening, and 2) it happened as part of a hardfork when people were trying to convince some big accounts to get out of the voting-bot business and switch to a "curation" business model, so the real goal may have just been to lower the impact of small votes and cause bigger votes to dominate even more than they already did so that that big accounts delivering big votes would have a more viable business model.