With clouds this is not true anymore. They are exactly linear. If you ask for a smaller node they are simply propositioning a chunk of a larger machine anyway.
There is a point where the exponential pricing starts, but that point is way out there than most people expect. Probably ~100CPU, ~1TB RAM, >50Gbps network etc.
They're linear... because they're charging you rates based on the cost of the large server, divided down into whatever server you provisioned.
Amusingly, for $94K (probably more like $85K after negotiation) you can buy a white box server: Dual Epyc 9000, 96 core/192thread, 3.5GHz, w/ 3TB RAM, 240T of very fast SSD, and a 10G NIC. The minimum config, Dual Epyc 9124, 16core/32thread, 64GB RAM, and only 4TB of storage is $9K (more like $8K after negotiation). That's "only" a factor of 10 in price for 8X CPUs, 48X the RAM, and 60X the storage.
There is a point where the exponential pricing starts, but that point is way out there than most people expect. Probably ~100CPU, ~1TB RAM, >50Gbps network etc.