After Alister became a Master Mage, the quality of his mana was greatly improved and the size of his mana core had expanded.
A mana core doesn't have a physical manifestation. So even if the mana core in Alister's body became bigger, it didn't make him look as if he was pregnant.
From Alister's conjectures, it was likely the rules of this world that allowed the creation of the mana core in another dimension inside the body of a mage.
It was like the so-called inner universe or the inner world in a cultivator's body.
However, from what Alister had known from Zanna, killing a mage would drop the mana core in their body which was also the reason why some mages would kill each other to obtain the mana core of their opponents and use forbidden magic spells to improve their power using it.
If a 10th-Circle Mage was powerful enough to destroy a kingdom in this world, then a Master Mage was powerful enough to destroy the continent.
