Alright, I was boosting the chars in the Dinosaur Forest, and while I was boosting Terra's Mgc. Pwr, she seemed to gain THREE Magic Points from a Magic+2 Esper (Tritoch).
I went "what the hell?", because I thought they never raised from just leveling up.
However, they must do, because I think one of my chars got 128 in Vigor AND Mgc. Pwr once, and that seems impossible from just boosting with Espers.