Hi,
From your layouts I see that full_adder1 in included in full_adder_box. As a consequence, it is normal the second layout is larger, because you added additional interconnection wires. The bounding box is the minimum box that contains the whole layout.
You con see a summary in the attached picture: bounding_box1 < bounding_box2 as expected.
Did you mean something different?
Fabrizio
