Very informative, thanks for the detailed response. To be clear, was my statement incorrect in terms of the "16%" quotation (from [0], which I now realize I misquoted in my private notes), or in the claim that the RSA key space density is low due to the dependency on prime numbers?

My private notes include a claim (original source uncited, sadly) that because the prime density from 1..n decreases (approximately) as `1/ln(n)`, then the corresponding effective key space density for RSA decreases at a related rate, which was what led to the corresponding reduction in the "equivalent" key size. If my notes on this are complete bunk I'd love to hear it. (Even a simple 'yes' would suffice and I'll revisit the subject.) Ta!

[0] https://www.yubico.com/2015/02/big-debate-2048-4096-yubicos-...