Cube's Behaviour Barely Affected By Mass


The problem that I’m facing is that no matter what I change in the Mass settings, my cubes keep bouncing like bouncy balls on the surface or when colliding with one another.

As for context: I set the size of all cubes to 1, and scale the cubes to the size I want them to be (think in the range of 200-600, e.g. x=300, y=500, z=200, where I imagine the sizes to be in mm, which is the unit that I use for all of the scaling in my scene). I give them the default Rigid Body and Collider, and add a Mass attribute. The thing is that I can put the mass on 10 and put it on 10000 (where I imagine the mass to be in kg), but the cubes keep bouncing like bouncy balls the same way. Too many zeros and the cubes fall through the other objects.

Now my question is: How do I properly give weights (in kg) to objects? And how do I take scaling into account in this process? Because I can imagine that different scaling with the same mass also changes the way an object behaves.
In general I just don’t know where to adjust things: in the rigid body, in the collidor, in the mass component, maybe even somewhere else?

Thank you in advance for the help!