Enter the Beta forum xD
Actually, it isn't, you are looking at it from half the space half the fps standpoint... Look at these stats:
CDM lvl 11-20
8043 fps
AO lvl 11-20
12752 fps
Now we come to the rub, CDM increases 5 fps for those 10 levels, AO 10 fps.
CDM= 8043/5
1608 fps per fp increase.
AO= 12752/10
1275 fps per fp increase.
While you can definitely argue the space issue -CDM takes up half the space- that ceases to be a real issue once you get above lvl 10. At that point it makes more sense to level AO above 10 than to level CDM above 10.