Here's why:

* Fuel Consumption: Stars generate energy through nuclear fusion, primarily fusing hydrogen into helium. Smaller stars burn their hydrogen fuel much more slowly and efficiently. Giant stars, with their larger masses, have much higher core temperatures and pressures, causing them to burn through their fuel at a much faster rate.

* Gravity and Fusion: The intense gravity of a large star forces its core to be much hotter and denser than the core of a smaller star. This leads to faster fusion reactions, consuming fuel more rapidly.

* Lifespan Calculation: The lifespan of a star is roughly proportional to the amount of fuel it has divided by the rate at which it consumes that fuel. Since smaller stars have less fuel but burn it much slower, their lifespan is significantly longer.

Example:

* A star like our Sun (a medium-sized star) has a lifespan of about 10 billion years.

* A giant star like Betelgeuse (a red supergiant) might only live for a few million years.

In summary, the smaller the star, the slower it burns its fuel and the longer it lives.