Quest XP is fixed per quest, is rewarded based on a multiplier, caps out at 5 levels above, and is limited at 10% of XP-to-level.

The multiplier is applied based on a quest's "difficulty", which the developers have hard-coded per quest:
Very Easy: x0.1
Easy: x0.75
Standard: x1
Hard: x1.5
Very Hard: x2.25
Note that the colours of quests in the journal is not related to this difficulty, and is a representation of your level relative to the quest level.

To explain the cap and limit, here's an example:

• At Level 9, completing a Level 9 Standard quest will give you the full amount of XP for a Level 9 Standard quest: 656 XP.
• At Level 9, completing a Level 9 Hard quest will give you the full amount of XP for a Level 9 Hard quest: 656 x 1.5 = 984 XP.
• Completing a Level 9 Very Hard quest should give you 656 x 2.25 = 1476 XP. However, at Level 9, the required XP to level up is 11,080 XP, so the quest will only reward 1108 XP (this is the 10% of XP-to-level limit).
• At Level 9, completing a Level 10 Standard quest will give you the full amount of XP for a Level 10 Standard quest: 670 XP.
• At Level 9, completing a Level 14 Standard quest will give you the full amount of XP for a Level 14 Standard quest: 1011 XP.
• At Level 9, completing a Level 15+ Standard quest will only give you the XP equivalent to a Level 14 Standard quest: 1011 XP (this is the 5 level cap).
• At any level, completing a quest that is a lower level than you are will reward all XP for that level of quest. Note that for quests that are grey in the journal, only the quest itself will reward XP, and you will not get any from enemies you kill.

Enemy XP is similar - you get full XP for any enemy killed within 5 levels of you, the XP is capped for enemies 5+ levels above you, and you get no XP for enemies more than 5 levels below you.