Why is the star Vega, at 2.1 times the Sun's mass, sometimes less dense overall than the Sun?

Immense outward pressure inflates the star to a larger radius

The relationship between mass and average density in massive main sequence stars can be counterintuitive. While the increased mass generates stronger gravity which should increase density, the concurrent increase in energy generation leads to immense outward radiation pressure. This pressure forces the star to inflate dramatically, expanding its physical radius significantly beyond what a moderate mass star experiences. In cases like Vega ($2.1 M_{ ext{Sun}}$), the increase in the physical radius outpaces the gain in mass, causing the overall bulk density of the star to be lower than that of the Sun, even though its core is far denser.