1. Long sleep or closed practice: During practice, the Monkey King may get fat due to long inactivity.

2. Overeating: the Monkey King may overeat for a while, leading to weight gain.

3. The influence of incarnation or magic transformation: In the sequel of The Journey to the West, the Monkey King may encounter some magic or transformation power, which causes his figure to change. Please note that the above are just some guesses, because The Journey to the West's sequel was not written by the original author Luo Guanzhong, and different sequels may have different creative interpretations. To understand why the Monkey King gained weight in the specific sequel, it is best to refer to the plot and story line of the related sequel.