Shouldn't the same feelings of smallness be imparted by staring at a sufficiently large building? So the sensation of awe fades as you become accustomed to seeing the object in question?
I cannot truly discuss the difference between awe-inspiring man-made objects and nature-made objects, but perhaps there is an implicit understanding of vastness with regard to nature that includes something intangible related to true complexity of nature: when viewing the Grand Canyon with more context you can try to experience the vastness in many variables: how long the canyon has existed, how long it took be carved out of the earth, how much water there once was flowing through it, etc. For a man-made object perhaps the context is more shallow.