Minecraft wiki entry for Pressure_Plate, associated text: The signal strength from a weighted pressure plate does not vary with the type of entity: all entities have the same "weight", so two mobs produce the same strength as two dropped items. An item stack counts as a single entity, no matter how many items are in the stack. When multiple items of the same type fall on a weighted pressure plate, they may initially be separate item entities and the weighted pressure plate counts them separately, but if they then collapse into a single item stack entity the signal strength from the weighted pressure plate may go down.