fluffrabbit {l Wrote}:A hierarchical scene tree is a trivial recursive function with matrix multiplication, so having "scene graph" as part of the name implies OSG does very little for the bloat it adds.
Don't forget that scene graph isn't just a hierarchy of transformations -- that really is the trivial stuff -- it's many other things like culling, inheritance of rendering attributes etc. These are non-trivial, because you e.g. have to group the nodes before rendering to minimize switching between states etc. It is kind of bloated... as most things written in C++... but it's still better than many other engines and good if you can really benefit from the scene graph. It's true though that a lot of people use it when a simpler engine would be more appropriate.
Irrlicht is a bit awkward with a lot of shortcomings, a lot of times it serves as a base for a custom fork rather than a complete solution (e.g. STK engine as you say). The main advantage of it is that it's pretty beginner friendly, but that is not what OP is looking for.