I'm pretty sure Punvaras are natural giantesses, they just are born smaller than humans as seedlings. He took the one with the most promise and basically juiced her up with magic MiracleGro.Even smaller? Oh wow, so it means that a this guy managed to not just strengthen her to a level similar to guardians (which is already near impossible), but also grown her from someone who wasn't supposed to be a giantess at all. That's not something that any giantess could do, and it was done by a human. That's why it's a plot hole for me - having human strong enough to do such thing breaks the rules of GG world.
That said, while Diula certainly is strong, Regina wasn't trying to hurt her, just keep her from hurting others while Diula was going all out, so there's a pretty wide gulf between the two still.