I know. Maybe if the idea was to prevent one from "abusing" it for easy XP, why not just make the XP gains fall as more kills of the given monster type are done (makes more sense when you think about it from a "realism" perspective too, after a while you've got the monster "all figured out" and so don't gain as much)?
Addendum: Hmm... looks like this might already be in there. Maybe tone it up more and tone down the monster-XP gain a good bit for the jackals as they're generated and summoned so frequently. It wouldn't be so bad if they weren't summoned so darn often (another option -- reduce the generation rate of jackal summoners?).