Another possibility is that there could be a wizard or a wise old man in the 'giant boy vore story' who becomes the main character. He could be the one guiding the giant boy to understand and deal with his 'vore' problem, using his magic or wisdom.
Maybe a small creature that was 'vored' by the giant boy could be the main character in a sense. It could tell the story from inside the boy, about its struggle to be noticed and set free.
The giant boy himself could be the main character, of course. He's the one with the unique situation of 'voring' things.
Well, it could be about a giant boy who accidentally swallows a small creature while exploring. Then he feels guilty and tries to find a way to reverse it.
It could be a regular boy named Tom. He's an ordinary kid with a normal life until one day he's struck by a strange ray that makes him giant. As he tries to deal with this new situation, he becomes the main character of this wild 'giant boy shrink story'.
Since we don't have much information, the main character could be an ordinary - looking boy from a small town. He might have a normal family, but something about him is special that leads to his growth. It could be his determination, his kindness, or some hidden talent that gets triggered and starts his growth journey, whether it's physical, mental, or emotional growth.
The main character could be a brave tiny woman who is not afraid to face the challenges related to vore. She might be a curious explorer, always looking for new things in a world full of strange and large creatures.
The fat boy and the giant boy are surely the main characters. However, there could be other supporting characters that we don't know about yet. Maybe there are friends or foes of the fat boy and the giant boy that play important roles in the story as well.
Often, the main characters are brave explorers. They are curious about the unknown and that's what leads them into the territory of the giant animals. For example, a group of jungle explorers might be the main characters in a story set in a tropical rainforest with giant beasts.
Maybe it's an alien being who has a different body structure and the 'big boobs grow giant' is a normal part of their growth cycle on Earth. This would make the story about how they adapt to Earth while also dealing with this growth and how humans react to it.
A thief might be the main character. They were trying to steal something from a strange laboratory when they accidentally triggered a device that shrunk them. Now they are in a world with a giant man. At first, they might try to steal from the giant man too, but as they get to know him, they have a change of heart and end up helping the giant man instead of causing more trouble.
The main characters are, of course, the giant and the little boy.
The main characters are the boy and the giant snake. The boy could be an ordinary child or someone with special qualities. The giant snake, on the other hand, could be a fearsome creature, but it may also have a complex personality. Their interactions, whether it's a conflict or an alliance, form the core of the story.