As the mass of the spheres increases, the gravitational force between them increases. According to Newton's law of universal gravitation, the gravitational force \( F \) between two masses \( m_1 \) and \( m_2 \) is given by the formula:
\[ F = G \frac{m_1 m_2}{r^2} \]
where \( G \) is the gravitational constant and \( r \) is the distance between the centers of the two masses. From this equation, you can see that if either or both of the masses \( m_1 \) or \( m_2 \) increase, the gravitational force \( F \) will also increase, assuming the distance \( r \) remains constant.