"How many legs does a dog have if you call the tail a leg?" Answer: "Four. Calling the tail a leg does not make it a leg."
"How big is a bounding box if you increase its size?" Answer: The size will be unchanged. If you were to change the size of the box, it wold no longer be the bounding box!
What you could do is take the coordinates of the bounding box and create new coordinates for a larger box. The last two coordinates are the height and width, so you could just change those, or you could also change the (x,y), depending on where, relative to the object, you want the expansion to be.
