Final answer:
The different size dots in the figure likely represent the luminosity of stars, with larger dots indicating more luminous stars. This is especially relevant in star clusters where all stars are approximately the same distance from Earth, allowing apparent brightness to accurately reflect luminosity.
Step-by-step explanation:
The different size dots in the figure most likely represent the luminosity of the stars, which is a property indicating how bright a star is. When stars are part of the same cluster and therefore roughly at the same distance from Earth, the apparent brightness (how bright they appear to us) is directly related to their luminosity. More luminous stars emit more light, and thus appear brighter, which can be represented in diagrams with larger dots.
Some additional references from the provided information hint at the size of the stars, such as the comparison between our Sun, Delta Boötis, and Xi Cygni. In these comparisons, the physically larger stars would also be depicted with larger dots to represent their size relative to others. Moreover, a graphic such as the H-R diagram (Hertzsprung-Russell diagram) classifies stars based on their luminosity and temperature, where luminosity could be visually represented by the size of dots on such a diagram.