Each pair of clay balls represents two planetesimals. If each planetesimal is composed of the same material and is separated he the same distance, which pair experiences the frayed gravitational attraction?

Every object that has mass exerts a gravitational pull on the objects around it. Depending on how big the object is, more the pull it exerts will be in magnitude. A massive object such as the Earth will inevitably overwhelm all other gravitational attraction caused by objects on it. In this case of planetesimals, even though the materials are the same, the mass must be different for one to experience a greater gravitational attraction than the other.

Hope that answers the question, have a great day!