MoGraph base scene sent via pm, however, here is a node based setup that can be used. It generates geometry (sphere) on contact points with floor. I used a simple comparison for performance purposes, you can use geometry collision if you will use some irregualr mesh such as terrain. Once the contact is made, the sphere grows to set size in duration set in Monoflop. Scene is commented. This would be the gist of your intended effect. Hope it helps
Keep_and_Grow_Collision_Value.c4d