Figure size?

[Michael Fournier]

One probable reason, I was told in art history class, for the seemingly small heads on the Sargent painting posted was: This painting is a full length very close to full size rendition that was intended to be hung with the subjects eyes at or about viewer's eye level or just slightly higher in a location that did not allow the viewer to stand back far enough to view the entire painting at once.

So the proportions as you look at the painting with the heads closest to you and the subjects feet way down by your own feet. The size of the head seems normal.

This is the opposite of what Michelangelo did with the David statute where the head and shoulders were intentionally over size so that when viewed looking up at the statue they would not appear too small. And the effect is a very heroic David.