NancyFx was also the first thing I thought off when looking at the examples. Before the .NET Core days I always used NancyFx as a drop-in REST service when I needed one in a non-web app setting, like a Windows service.
Personally I don't use macro/micro to describe the bytecode footprint, but rather the API/conceptual footprint. If Jetty is secure and performant, and Javalin keeps it encapsulated, I think micro is a fair descriptor still.