My absolute favorite inflation idea is contagious inflation. As in multiple people end up inflating and it spreads to other people.
Example one: Girl drinks tainted milk, girl starts filling with up with milk, girl leaks milk or pops, (Personally I prefer non harmful/fatal popping) other people hit by the milk start inflating too.
Example two: Person inflates, pops, releases chemical or gas that causes others who breathe it to also inflate.
This also leads to another idea I love, multiple people inflating in one space. Pressing up against one another and squeezing together as they quickly outgrow the space.