An 1847 manual of oil painting explains: “When the lights of a painting are so arranged that they seem to be in masses, and the darks are massed to support them, we have what is called breadth of effect, which is mainly produced by the coloring and chiaroscuro."
The painting "Blessing of the Young Couple Before Marriage" by Pascal Dagnan Bouveret is a good example of breadth. The white of the bride's dress joins with the table cloth, the shaft of light, and the other women's dresses to make a single large shape. Meanwhile, two groupings of dark-clad figures join to form larger masses.
The painting manual says that the quality of breadth applies to both design and coloring, and that it is indicative of a master. Indeed it's difficult to achieve, because one must overcome the natural inclination to separate and define shapes throughout the composition.