the thing about Hard reference vs soft reference has to do with the amount of memory consumed by Default_Objects (the template in which the instance objects are created from)
so if you mean that the 10MB number comes from the Size map that is the size on Disk of the Particle system that will be loaded into memory when anything that has a hard reference to that Particle system is loaded (or is when the Particle system is loaded into memory through the Asset Manager.
so that 10MB number will only occur once
for how much memory each instance of these Particle System takes that will be dependent on the amount of compute on the GPU that the Particle system needs. Particle Systems live on the GPU not system memory (mainly because they are graphics things, and they don’t typically interact with physics, or gameplay logic.
you can actually look up videos of like Enemy Randomize mods for games to see what effect multiple instance of a particle system can have on performance.
to determine the overall result of your Particle system would be done through Profiling the application (again for Particle system this should impact the GPU not the CPU
what I would summize you will see is the Particle system will be 10MB in size, with each reference to the Particle system would be 64bits (unless you are forcing the engine into 32bit mode), then the Transform of the Particle system (10-doubles and functions for the `FTransform) meaning less then a MB of system memory for each.