Fear of the Unknown
When Lan had enlarged his body, he had suddenly crushed the stones that made up the roads; cracks ran around the whole street, even the houses had cracks traveling up on the walls, which showed how heavy his body had been, and how severe the damage done to the foundation of the road was.
The ghosts, which had seen this sudden transformation, had been so shocked that they had retreated rapidly, not wishing to be crushed beneath Lan's body. While they were retreating, they were also beginning to plan their options for regaining the advantage.
At that time, Lan was suddenly gone. He had shrunk to a tiny shape once more, and went after Xue Wei, who had begun retreating the moment the snake expanded.
Now that he had returned to his normal shape, Xue Wei had already gotten far away, and the two were gone from the sight of the present ghosts before they managed to react.