Unity Essentials Course: Rigidbodies falling on structure,Unity Essentials Question: RigidBody objects falling on structure

Hi,

I have a question about an issue with RigidBody primitives falling on a structure of different stairs, columns, and platforms which are all nested in an empty parent gameobject. This project is one of the starter unity essentials course projects. The issue is that the original empty game object that is parent to the structure has its own transform of (1,1,1). The structure within built relative to the empty object takes up a lot less space, leaving an empty space in between the structure and the surrounding boundary of the game object which is in the shape of a sphere. All rigidbody components that are positioned in mid-air above the structure fall and collide with the outer boundary of the empty gameobject instead of falling through it to the structure. The empty object has empty space in between its outer boundary at (1,1,1) and the child structure, so when I render the scene, all rigidbody objects falls and hit the parent and then roll off its sides. Visually it looks like a bunch of objects hitting an invisible barrier well above the physical structure, and then rolling off on the sides of it. However, there are no components associated with the parent object other than transform, so I don't understand why my rigidbodies would collide with it at all. The essentials project doesn't suggest that this could be an issue, it assumes the objects will fall right onto the structure and bounce down it. Any suggestions for what might be happening?