In this story, the journey of the water droplet is a fictional story, and there is no specific scientific or logical basis. Therefore, we can explain why Mama Yun suddenly became fat according to the description of the story.

In the story, small water droplets were taken into the sky by the sun father-in-law, turned into countless small water droplets, and gathered into clouds. When the journey of water drops continued, they met grandma Feng, who blew them together. The more they blow, the bigger the clouds become.

Therefore, the reason why Mama Yun became fat was because the small water droplets were blown together by the wind mother-in-law and gathered into a bigger cloud, which made Mama Yun look fat. This is just a fictional story, which can't be compared with meteorological science in real life.