Hi Micha- the display in Rhino’s default shaded or rendered viewport uses smoothing tricks to make these look smooth- you can see the actual mesh facets, which is the real shape, by setting the viewport to use Flat Shading- _FlatShade command.
I’d suggest using Grasshopper and the Weaverbird add-on for Grasshopper to sub-divide the mesh. The workflow I used here was to select one of the humans>Explode>Join>Assign to a mesh component in GH>Sub-divide with Weaverbird>Bake the result back to Rhino for export as stl.