Is the Sun shrinking ?

[Chronos]

At any given temperature and pressure 2 moles of hydrogen will take up twice as much volume as 1 mole of helium.
Now the core of a star is a somewhat extreme environment in which to apply the ideal gas law, but even more complex versions of PV = nRT follow the same general trend linking volume, moles, pressure and temperature.

4 moles of hydrogen to 1 of helium. At those temperatures, everything is monatomic.