phy_mass;
Returns: Real
This read-only variable returns the mass of the instance
in kilograms. This value is calculated automatically based
on the surface area of the assigned fixtures and their density
values, but it can be changed using the function physics_mass_properties.
if phy_mass < other.phy_mass
{
instance_destroy();
}
The above code is from the collision event of the instance with another and it compares the mass of each instance and destroys that which has less mass.