Rested XP is always a %.
One bar is earned every 8 hours while logged out in an Inn.
If you needed, say, 100 XP for a level, you gain 20 Rested XP in 8 hours.
If you needed, say, 1000 XP for a level, you gain 200 rested XP in 8 hours.
Considering that the XP needed per level has jumped significantly (nearly *doubling* from 84 to 85), it would indeed seem like it was buffed even though it was not.
Edit: Relevant link:
http://www.wowpedia.org/Rested
Edited, Dec 20th 2010 5:21pm by Lyrailis