Scale is supposed to make things more efficient, not less. That's the whole idea behind "economies of scale".
Sure, if you want to pretend the EU doesn't exist, and that the US doesn't already manage Medicaid on a state-by-state level, there's a significant population difference... but Germany has nearly 100M people. We have strong evidence single-payer healthcare already works with large numbers of people.
Most of the comparisons that people like to bring up are European countries the size and population of US states.