"I was experimenting with a new character - at level 8 I killed a mountain giant and got nearly 500 XP - at lvl 10 the XP was down to about 300 and by level 12 well under 200."
I'm sorry what Barry, I am calling you out on that one instantly, look at the third imgur picture that is a level 10 character no mulitclassing, getting 151 Exp from a Hill Giant Bruiser.