Depends on the strength of the particular Capital and the level of Discipline really.  Some of the higher ones are practically untouchable, I have experience the same thing sending to level 12+ capitals, you will not score anything unless you send mega armies and even then, I think the kill ratio will be extremely skewed.

If you send small groups of Units, you're unlikely to earn any points or kill anything in a Capital. 

i am not sure how percizely is formula but it is something like this:

sending 50k to capital with let say 100 mil defense will end up in total loss

the reason is that game divides atack on def and calculates percentage of loss to attacker and deffender

so 50k / 100 000k = 0.05% 

so suiciding attack with 50 k will suffer 99.95%  or practicly all

deffender will loose 0.05% or practicaly null

PS: On persians percentage system is not active and its like 50k attack kills 50 k def allways

i am not sure how percizely is formula but it is something like this:

sending 50k to capital with let say 100 mil defense will end up in total loss

the reason is that game divides atack on def and calculates percentage of loss to attacker and deffender

so 50k / 100 000k = 0.05% 

so suiciding attack with 50 k will suffer 99.95%  or practicly all

deffender will loose 0.05% or practicaly null

PS: On persians percentage system is not active and its like 50k attack kills 50 k def allways