In any circuit voltage is the key too much and things burn up..too little things dont work. the weakest link is the component that is the most voltage sensitive. For example if your running HV servos it doesnt mean you can put 7.4 to the receiver.
as far as capacity goes electricity is lazy. it only gives you what it takes to do the job. i used to have a starter that I hooked up to a car jump starter, 300 amp of cranking power. now 300 amps would have done damage to me and the starter...but it only gives what the starter needed.
ok so if your airplane pulls 10mA on a 2000 mAh battery and you change nothing except the battery, go say 3000 mAh. you have more to pull from.
I hope I answered your question