In Dragon Warrior 3 (GBC):
In one level up, my hero gained 316 MP.
What ? Do you have any idea why ? A glitch ? =)
It's not a glitch, but rather a quirk of the way level up works, and the result of a couple hours of work.
The thing is, in this game, when you level up, the game calculates your new MP based off your INT score; the game takes your INT, multiplies it by 2, applies a small random factor (I believe up to 2.5% in either direction), and if the new result is higher than your old maximum MP, that becomes your new max MP total. HP is handled similarly, but is based off your VIT. (There's one other rule: A character in a non-spellcasting class (Warrior of Fighter) will never gain MP at level up, even if the character has spells due to having class changed from a different class.)
As for the couple hours of work: I took a party of 4 thieves (after beating the game, you can remove the hero from your party), went to the Tower of Garuna, and killed Great Beaks. This enemy can drop the INTseed, which is a consumable item that permanently raises a character's INT by 1-3 points, and having thieves in the party grants extra chances (at higher probability) of getting the drop. Hence, I was able to, after a while, get enough INTseeds to raise my Hero's INT from 103 (IIRC) to 255 (the maximum).
Then, all I had to do was feed my hero the INTseeds to max her INT, and then level her up once. She had 201 MP before the level up, and the game rolled 517, so she gained 316 MP from that one level up.
Note that this strategy does *not* work in the NES version; MP (and HP) gains are calculated differently. The only way to gain MP in those versions is to gain INT at level up, at which point you gain MP equal to 2x the INT gain, plus or minus two. In fact, not only do INT seeds not give you MP, but they can actually hurt your MP growth, by putting the stat closer to the soft cap (which scales with level and prevents stats from growing too high too soon, especially after class change) and to the hard cap of 255. It does work in other remakes of the game (SFC and mobile); the one strategic difference is that you need to kill the Great Beak last in the GBC version, but not in the other remakes.
It's also interesting to note that HP gain in classic Wizardry games (and in a few related games, including Elminage Gothic and apparently Wizards and Warriors (PC)) uses a similar mechanic; HP is rolled based of class, level, and Vitality, and that becomes your new max HP if it's higher than the old value, otherwise you only gain 1 hit point.