Your gravity wakes up
Cross a kilometer or so and something changes for good. You are now massive enough that your own gravity reaches out and bends the paths of things near you. You are a planetesimal, a true building block of worlds, and you no longer wait for the universe to drift past. You pull it in.
Runaway growth
Gravity rewards the biggest. The largest planetesimal in a neighborhood pulls hardest, so it grows fastest, so it pulls harder still. This is runaway growth, and it is how a few bodies race ahead of the millions around them, eating their smaller siblings and clearing room to become something more.