|Porcelain lettuce and a cut glass decanter|
Too many times I see this clustering concept fail, because people tend to want things to be all the same size, or all the same colour, or evenly spaced. Take a page from Mother Nature's book and stop trying to make everything the same!
In this case it's all about the size of the objects. Both are delicate in design but solid in shape, neither object overpowers the other and thus they are complementary. They also work in this narrow space because they fit within it, things hanging over the edges of shelves or mantles look messy and precarious. The decanter 'ends' the space with its height, giving the mantle a softer look- not everything can make a big statement. The decanter also catches the light, and an incidental effect is the pattern created on the wall behind it.
Today's interior decorating lesson is: either complement or contrast because you can't do both!