Why would you assume the population cap is arbitrary? There's a calculable limit to the population an area of land can sustain. (Yes, some agricultural practices can mitigate that, but that should also be weighed against culture and history, and how much change is acceptable.)

https://en.wikipedia.org/wiki/Carrying_capacity

Ok, so how to calculate it for switzerland in a non arbitrary way?

(Btw. I believe switzerland is not trying to be self sufficient anyway, but donimport lots of stuff, like most other countries do)