Did a little digging in the patch notes. This is what I found:
2/12/2008: Enemy XP values (and Influence/Infamy dropped) have been increased across nearly all levels. The size of the increase directly relates to datamined leveling speeds for characters, so levels where characters have found themselves ‘stuck’ or ‘slowed’ will have enemies of that level’s XP increased more than other levels. Enemy Levels with the greatest increase in XP rates are: 13 to 20, and 36 to 50. Other levels have had smaller, but still significant, XP and Influence adjustments. This may mean that characters will find it impossible to do ‘all’ the content of a given level range before outleveling it.
http://web.archive.org/web/20121003093848/http://na.cityofheroes.com/en/news/game_updates/issue_11/xp_curve_smoothing_qa.php
9/15/2009: To go along with our new changes to sidekicking and exemplaring, we've made some adjustments to the experience awarded from minions, lieutenants and bosses in the level 5 to 24 range. In short these enemies are on average receiving a 20% experience and influence increase. This will slightly speed up the pace at which heroes and villains advance through the 5 to 24 range.
So it appears that xp rewards were buffed at least twice.