Here I can see that Octane is using around 15GB of RAM itself for rendering, near all of it used by the Octane core to build the acceleration structures, AI scene cache, etc. Probably one of the reasons why Octane needs so much RAM is due to the small size of the final triangles. The object is made using quads, so finally, you have around 22.9M triangles and the final GPU memory used for rendering is 3.7GB, as you can see in the attached image.
I suppose that this is the expected Octane behavior, but in any case, even with 32GB of CPU RAM you should have no problem to render objects like that. Mantra probably doesn't build so complex acceleration and cache structures, but well, in the end, thanks to all this additional data Octane can be a lot faster