Human-robot co-grasping allows the collaborative transportation of large objects in industrial and domestic settings and represents an important challenge in the research area of physical Human-Robot Interaction (pHRI). Here we propose a method that allows the co-manipulation of large objects by considering an extended contact patch between the robot end- effector and the object. Pressure and friction constraints are derived from the geometrical properties of the contact area and are at the basis of a control strategy which allows the robot to maintain the grasp stable while following the human motion.