It absolutely varies a lot within Europe too, but my feeling at least is that the difference between European and US portion sizes gets bigger as you move towar

It absolutely varies a lot within Europe too, but my feeling at least is that the difference between European and US portion sizes gets bigger as you move towards low-end places. High-end restaurants are pretty similar in portion sizes almost everywhere I've been, presumably because they're not competing on portion sizes, while lower-end places are much more susceptible to local expectations of what is good value.