A 10kg projectile moves a 1 million ton object.

Has anyone noticed this strange behavior? I’ve been working on physics based projectile behavior for a while now, but am plagued by an issue where I am sometimes displacing much heavier objects too easily.
Yes, this is not a friction issue. Even if there was no friction, it would be impossible for a 10 kg object to move a 1 million ton object. This is impossible even in zero gravity.
But sometimes, the game engine seems to forget the weight of the objects it is simulating. I don’t know the conditions, an object set to 1 million tonnes will sometimes behave like 100kg (This is an extreme setting, but I’ve also tried it with more realistic weights). If a 10kg object hits it, it will be pushed several meters away, no matter how heavy the object is.

So far the only physics setting I’ve changed is to enable substeps. Does anyone know anything about this issue?