Actually capacity is an issue depending on how you plan to use the battery.
Small capacity batteries can only supply low amperage, and you will cap the output fairly quickly if you are planning on using a small battery to say power a tuned AEG.
Plugging an AEG into a battery that can not supply enough amps will cause the battery to overheat. I would not expect any lipo pack to have reliable thermal protection. An 18650 does in the form of a fuse, but a lipo pack is a different animal without the fused top cap.
Capacity and discharge (C) work together to give you the amperage of the battery.
Take a random battery (invented for demonstration).
7.4v 2S 500mAh 15/30C
- 500mAh is 0.5Ah.
- 0.5Ah x 15C = 7.5amps continuous discharge
- 0.5Ah x 30C = 15amps burst discharge
If you plugged that into a stock AEG pulling 10-15amps the chance of an overheating and potentially thermal runaway battery would be high. Dangerously so.
A typical airsoft AEG battery looks more like this.
7.4v 2S 1300mAh 15/30C
- 1300mAh is 1.3Ah.
- 1.3Ah x 15C = 19.5 amps continuous
- 1.3Ah x 30X = 39 amps burst.
The continuous discharge of 19.5 is over the 15amp draw rate, meaning whilst it'll get hot, it should be safe from thermal runaway.
If you are trying to make a performance AEG with uprated motors and ROF then be even more vigilant to amp draw. You simply can't make an M4 pistol that does 25rps, and run it on anaemic batteries, You'll be throwing a pile of burning plastic on the floor after a few bursts.
There are ways around small battery spaces. Tell us what the AEG is, Show us all a picture ect. And people on here will have some form of safe solution that works.
I am not trying to build a monster 
