In my experience that's just the leveling curve of this game. At low level/rank the sp/exp gain is really high.
And as you rank up it becomes less unless you kill things which are about 5 ranks higher than yourself.
My suggestion would be to go to Ul'dah and shout/seek for a party. In a party I find you can get a decent 500sp/kill
from mobs which are about 10 ranks higher than yourself.