Is there a technical reason that new (Apple Silicon) Macs need less RAM?

I typically don't upgrade my MacBook Pro until I can buy a machine with at least twice the RAM. (I rarely find the CPU to be the primary limitation of any machine I use.) When Apple released a MacBook Pro with 64 GB I was interested in getting it, but it would have totaled close to $4000, so I held off and kept using my 16 GB MBP from 2015. Now my current machine is having issues that I've been unable to resolve, so I'm in a hurry to replace it. However if I want to buy anything with the new "Apple Silicon" M1 chip, it will max out at 16 GB, and anything more will require buying a legacy CPU, which seems ridiculous at this point.

Is there some reason that the newer machines can do more with the same amount of RAM? Are they somehow more efficient with memory? Or is this simply part of rolling out low-end machines first, with no particular technical justification?

The 'technical justification' is that the RAM in the Apple Silicon chips is part of the chip itself, it is not a separate entity like all other processors before.

Presumably, they'll perfect how to enlarge this with successive generations.

The broad strokes of how they do this are on the announcement page - https://www.apple.com/mac/m1/

You could probably consider this first generation to be a more than adequate 'proof of concept'.

You will be fine with 16 GB of ram. It will not feel like your old machine. Take this for example: The best selling Android has 12 GB of ram The iPhone 12 Pro has just 6GB of ram The iPhone is MUCH faster and has better performance. Okay back to your question: because of the tighter integration between your Mac, M1, and the manufacturing, you should be fine with 16 GB.