There are some personal quests, which only give experience to the player who completes them, in additional to party quests, which give experience to all.
How many personal quests there are, and (easier to adjust) how much experience is available from them, is a subject to balancing changes. In fact, once the tutorial is in and possibly some other content, there will adjustments made to experience (quest, exploration and combat) throughout the first act.
FWIW, the amount of XP required to level goes up as the cube of the level, so while a third of a level may seem like a lot at level 3, that experience difference quickly becomes much less significant.
Regardless, this issue is on the feedback list, particularly with co-op playthroughs where one person does most of the talking.
