Does playing a low-level quest in Diablo III give me less XP per kill?

Presumably, the reason they're easy is because you've outleveled them. Monsters below your level provide less experience, as seen in the table below.

Level difference is calculated as (mob level) - (character level).

Level difference  Multiplier 
+3 or more        125% 
+2                120% 
+1                115% 
 0                100% 
-1                 90% 
-2                 80% 
-3                 70% 
-4                 60% 
-5                 45% 
-6                 30%
-7                 15%
-8                  5%
-9                  1%
-10 or below        0%

Bonus XP/kill from gear is only applied if the monster is at most 5 levels below you.

Source: Click to Loot