Due to its density, the white dwarf steals mass from the fluffy, less dense star, producing an accretion disk around itself. This way, the small white dwarf gains slowly more and more mass, until it reaches a critical mass limit, known as the Chandrasekhar limit in honor of its discoverer. The deal is that at this limit the “repulsive” pressure exerted by the electrons of the white dwarf no longer resists the extra gravitational push of the extra mass gained.