Why are main sequence stars more massive than the Sun less dense? e.g. Vega, Spica etc Why do main sequence stars more massive than the Sun have lower densities? e.g. Vega, Spica etc.
Main sequence stars are defined by being hot enough in the core to fuse hydrogen, so their core is at least about 10 million K, and can get up to 20 million K for the more massive ones (because they are more luminous, so their nuclear fusion has to crank itself up a bit more).   

The way they get their cores hot enough to fuse hydrogen is via gravitational contraction, but a more massive star does not need to contract as much to reach the necessary temperature.   Since it does not need to contract as much, it is at lower density.  

To see the scaling approximately, use the virial theorem, which says that the energy per particle in the core (roughly set by the need for fusion) is about $GMm/R$, where $M$ is the mass of the star, and $m$ is the mass of a proton.  Expressed in terms of density $\rho$ instead of radius $R$, that's proportional to $M^{\frac{2}{3}}\rho^{\frac{1}{3}}$.  Ergo, $\rho$ drops when $M$ rises.