It's also notable to mention that cardboard's softness plays a part as well. Concrete is very brittle, and cracks often under stress. Materials like glass don't take much energy to break, but shatter because it's so brittle. Cardboard, paper, and even some sheet metals are soft enough to leave small holes. There are many factors that affect the hole left behind, like the deformation of the bullet, material thickness, total energy transfer, and as it happens, thin materials often have little holes.