Hotter, not necessarily larger
A blue supergiant can have a radius tens of times the Sun rather than hundreds, but its high temperature makes it incredibly bright.
Because luminosity scales with temperature to the fourth power, a hotter surface changes the total energy output dramatically.
Evolution is not one-way
Massive stars can move across the Hertzsprung-Russell diagram. Some may appear as blue supergiants before or after red-supergiant phases, depending on mass loss and internal mixing.
This is why a neat classroom sequence is useful, but real stars are messier.
Why they matter
Blue supergiants shape their surroundings with ultraviolet light and stellar winds. They are visible across large distances and can help astronomers study stellar evolution in other galaxies.