The hottest most luminous stars are the most rare, while very low luminosity, cool stars are the most common in the Galaxy.
About 90% of stars in the sky seem to lie along this diagonal line called the main sequence. Stars found along the main sequence are also called dwarf stars because they are the smallest of their temperature class.
Three other important types of stars are seen:
Giants, most typically as Red Giants. These stars have radii typically 10 to 100 times greater than the dwarf stars of the same temperature.
Super Giants which have radii up to 1000 times that of main sequence dwarf stars.
White dwarfs These are a special class of objects which are extremely HOT, but very dim, and thus are very, very small. They are not true stars.
Much of what is happening on the HR diagram can be understand by considering this single equation:
Note, the range of stellar radii on the main sequence is not really large. See here