It's like how there are small Sea Kings, and larger Sea Kings.
I think the Oni used to be massive, and that Joy Boy was a continent creator, rather than a continent-mover like Oars. This is why he was known as a liberator: Joy Boy would create new islands by moving the earth itself, because he just had that much big-chungus energy.
Oars Jr was a bit smaller than Oars, so I assume their ancestors were just massive.
The only way something could maintain that size would be to hunt Sea Kings, IMO. So, if those are Giant Oni, then they have probably been eating Sea Kings out of the ocean to survive.