In this world, fairies can change their size if need be, but only if they have space. So jars can contain them and can reproduce with humans

He put both hands on either side of the chair you were tied to. "WHERE THE HELL ARE THE FAIRIES YOU RELEASED!"

"How should I know? They're free now," you told him

He grabbed a switchblade and held it against your neck. "You'll be a rotting corpse by next week if you don't bring back my fairies," he told you