Where are the giants, supergiants and hypergiants?
Follow along the horizontal curves (highlighted in red). These are the locations of the giant stars. Giants don't behave like main sequence stars. The brightness is constant or even increasing as you move toward cooler temperatures (from blue toward red).
If the sizes of the stars were not changing, then as we learned earlier, the brightness should drop as the temperature drops. To keep this from happening, the stars must increase rapidly in size as we move along the giant branches toward red.
In fact, the red hypergiants are the largest known stars. Some of these would overfill the orbit of Saturn, if they were placed at the center of the solar system.