Here's why:

* Luminosity is related to a star's energy output. Larger stars have more mass, meaning they have stronger gravity pulling more material into their core. This leads to:

* Higher core temperature and pressure: This allows for faster nuclear fusion rates.

* Greater energy production: More fusion means more energy released as light and heat.

* Smaller stars burn their fuel more slowly: While they produce less energy per unit of time, they have a much longer lifespan than massive stars.

In summary:

* Massive stars are brighter and hotter, but have shorter lifespans.

* Smaller stars are dimmer and cooler, but have much longer lifespans.