stat gain

How are stats decided as levels increase? I understand that stats change with jobs, but is there a certain distinction as to how they grow? Is it random, or preset, or maybe based on what job you are when you level or what abilities you use? My Bartz is level 80 right now and his highest stat is Mag. by 7 points. I found this odd because his high stat at the beginning of the game is vigor. But I've been using odin and bahamut constantly in my training, so I thought that might've been boosting the stat. Please help.