Hey there
The mechanics are just fine, two objects no matter what the mass/weight is, will fall at exactly the same speed to the larger gravitational object. In a perfect vacuum a feather will fall just as fast as a cannon ball. In your example, because the atmospheric density is so negligable, for a smoothly shaped object,there would be no difference between 100000kg, or 100kg.
I really don’t know a way of setting the Air density/pressure for a given height in UE4.
Hope this helps,