The volume of a sphere with a 3km radius is 113.1 km3; the volume of a sphere with a 30km radius is 113,097 km3. That would make it more like 1,000 times the volume.
10 x volume (1131 km3) would be a radius of 6.46 km.
The volume is irrelevant -- what is important is the force due to the mass in the black hole and the escape velocity. After a bit of algebra, you obtain the Schwarzschild radius, where R = 2 * G * Mass * c^-2. Therefore, a 10-solar-mass black hole has a radius 10 times greater than a 1-solar-mass black hole.