Yeah, but while Belgium is basically a huge plain, Switzerland is 60% Alps.

If you account for that, the effective density of Switzerland on the usable area is 600–700 people/km².

Looks like it’s 380 in the Swiss Plateau (you might be mixing up sq km and sq mi), which puts its at about ~70% of the population density of the Netherlands as a whole.

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