Final answer:
A star in the upper right of the Hertzsprung-Russell diagram is larger than a star in the middle because it is cooler yet more luminous, indicating it has a vast surface area, characteristic of giants or supergiants.
Step-by-step explanation:
According to the Hertzsprung-Russell diagram, a map plotting stars' surface temperature against their luminosity, most stars fall within the main sequence. However, some are positioned differently, such as those in the upper right corner of the diagram. A star found in the upper right part of the H-R diagram is characterized by a combination of low temperature and high luminosity, making it cool and luminous at the same time. This is only possible if the star is significantly large, hence why stars in this area of the diagram are typically giants or supergiants, with a very large surface area to compensate for the lower temperature per square meter. Therefore, compared to a star in the middle of the H-R diagram, a star in the upper right is larger.