Then you didn't do all vanilla content. You get XP for discovering areas, solving quests, picking locks, disarming traps, and Od Nua is full of all those things.
Doing all vanilla content before Court of the Penitents will get you up to level 13 and some change. Completing a bit of expansion content will then bump you up to level 14 quite quickly. In my full playthrough in 2.01, this meant I did most of the expansion content with max level, as well as the final campaign battle.
In my opinion the last 5 levels or so were gained too quickly, so I'd appreciate a ~15-20% increase to experience needed to gain those last 5 levels.