If you attach a child to a parent, the actually Bound of “Whole Actor” will be the green box :
so if you attach two children, the bound will be :
there is a lot of Blank Space, so if you try “Sweep” check, it may be weird.
Also, if they use “Convex” bound for Sweep Testing, means all components have own complicate collisions and combine together, can cause more calculation than only one bound and the result may not be what you want.
I am not 100% sure if I am right (Just my experience). Because it is about the purpose that engine designer want to achieve.