IIRC, the quest EXP you get is determined via the difference in your tamer level and the tamer level appropiate for the quest.
I.E.: Let's say you grab a Tamer 60 quest. If you're Tamer 60 - 63 getting that quest, you get the maximum quest EXP. If you're Tamer 64 - 66, you get a slight penalty on the given EXP. Tamer 67 - 70, you'll get even much lesser EXP. Once you're 10 tamer levels off the quest, you'll just get the Bits.