It was soon realized that the gas inside a white dwarf was too dense to behave as an ideal gas and, instead, was degenerate. For normal stars, if you increase the mass, the star gets larger, its radius increases. However, for white dwarfs, the opposite is true, increasing the mass shrinks the star. Notice that at some mass the radius of the star goes to zero. The size of a star is a balance between pressure and gravity.
Gravity pulls the outer layers of the star inward.
Pressure pushes those layers upward. But increased density does increase gravity. So, as you add mass to a white dwarf, the gravity increases, but the pressure only changes a small amount. Gravity wins and the star shrinks. Notice that the mass-radius relation for white dwarfs means you cannot keep adding mass to a star, for eventually its radius goes to zero.