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Simply put, Secondary quests, Treasure hunts and Witcher Contracts are optional side quests.
That's the reason main quests give you plenty of XPs.
To balance this when you overlevel too much a quest you get just 5 XPs from completing it.
Take note that it's still possible to receive more XPs doing the quest.

If you use Axii in the dialogue options you'll get 40 XPs or so each time you use it.