If your chest is too big for the corset you will probably experience spilling. This is caused by the excess tissue which is not able to fit into the corset cup and which is hanging over your "natural chest". Sometimes you can have spilling only at the sides (which can be caused by a lesser extinct by large cup sizes as well, but disappears from readjusting the corset), sometimes it covers your whole chest. Not only does this look unappealing, but wearing a too small corset for your chest can hurt your tissue and cause discomfort. If possible, buy the same corset size with a larger cup, otherwise don't buy this corset!
While all in all, fitting the chest of an over bust corset might not be as important as fitting the bodice (which might cause discomfort and bruising), it is still an important part for the overall fit.