Here's why:

* Luminosity: Stars in the upper right corner are the most luminous, meaning they emit the most light and energy.

* Temperature: They are also relatively cool, with surface temperatures around 3,000 to 5,000 Kelvin.

* Size: Since they are cool but emit a lot of energy, they must be very large to compensate for their low temperature. These are known as red giants and supergiants.

Examples of large stars in the HR Diagram:

* Betelgeuse (in the constellation Orion) is a red supergiant and one of the largest known stars.

* Antares (in the constellation Scorpius) is another red supergiant, also incredibly large.

* Arcturus (in the constellation Boötes) is a red giant star, although not as large as supergiants.

It's important to note that the HR Diagram plots luminosity against surface temperature, not size directly. However, the relationship between these factors allows us to deduce the relative size of stars based on their position on the diagram.