Here are some of my favorite suggestions:
Her body being inflated buy evil hands, Causing her ass to get massive. Before she could strike in revenge. A piston comes down flattening most of her body except for her lips, eyes, boobs and especially her ass which bulge out even more from the pressure of the piston.
---
Art of Expansion: Rewatching episodes of Naruto, Mai gets an idea to try out the ninjitsu skills of Choji and his family. Each attempt to replicate it has a different result, head, belly, butt, upper half, lower half, before reaching Uber inflation.
---
Someone stuffs an air pump in her mouth and starts inflating her. As she swallows, it travels from her breasts, to her belly, then her ass, until her entire body is inflated.
---
Mai is walking home after a long day of fighting, on her way home she finds a balloon stand, she sees a helium tank and discides to have a little fun she puts the hose in her mouth then her breasts starts to inflate, she loves the feeling of it and cranks the valve to max, this then inflates her butt and knocks over the helium tank, the valve breaks off and inflates her into Uber, next she can't take anymore and pops, she's then on the ground next to the helium tank knocked out.
---
Soda of Fury: Mai is enjoying her time in the kingdom in a soda fountain tasting some of the beverages while being careful in how much she drinks due how carbonated they are, that is until the evil hands show up and begin to force all the soda is available until they are cut short due the supply being too little but is more than enough that they begin to shake Mai wildly and effects of the soda begin to take effect and Mai's breast and butt begin to expand but seeing that is taking too long to continue to shake until all air in her is distributed to all her body until she ends up in a uber state.
---
The evil hands fills mai with tons of water giving her a bottom heavy look, in anger she prepare to use a immense fire attack. However the intense heat of the flame turns the water in her body into steam and Mai uber-inflates.