If you are realizing this because you're trying to level a class that you didn't start with, they actually get bonus exp in relation to the level gap between the side class and your main class. This bonus experience only applies to slaying monsters and doing Fates though, and this is done specifically so that veteran players don't completely ignore Fate events and so that they actually succeed most of the time. The random nature of Fate events make it difficult to farm though, making it so that doing Hunting Logs and completing quests are still a far better use of time while a Fate event isn't near.