To get this trophy you need to stretch BOY out a lot by holdingL3 and R3 in seperate directions, and then eat 20 inhabitants with L2. The twenty inhabitants have to be in his stomach all at once, but both people and animals count for this. You can eat other things accidentally too and it will not void the trophy, I know I ate at least one unicycle when I got it. After you get it you can just hold R2 to empty BOY and click in L3 and R3 to shrink him back down. Also, if you notice BOY is farting out inhabitants then just stretch him a bit more and eat then again.
As a side note, you should also get
And it's also a great time to start trying for
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