Hi Joao,
There will be memory differences in the situation you have described due to the ancillary data needed for the physics. You are correct that mesh data is essentially shared between instances for the same types, but there will be unique parts as well - so for example the transform meshes will differ, and they will have unique data in the acceleration structures as well, the solver will have unique data etc.
Best
Geoff Stacey
Developer Relations
EPIC Games