I'm not too sure what you mean but i guess you are referring to the many monsters outside of orario. Which are offspring of original (aka being born in the dungeon examples).
Basically most monsters outside of orario have weakened considerably because they divided their magic stones everytime they 'had children'. As such the monster in the outside world are on average a lot weaker then the ones in the dungeon. As far as i know they aren't smaller then the ones in the dungeon though (i could be wrong about that).
And yes the leviathan and one eyed black dragon and such are original monster. They just escaped a long time ago.
btw they explain some of this stuff in quest x quest a small story included in volume 04 of the main series.