The basic theory is that a bigger battery is always going to be better, because of the voltage drop effect.
In a simple device with no electronics, a small battery suffers from a bigger voltage drop than a large battery does, when supplying a comparatively high load for this size range of batteries. An ecig of any kind puts an extremely high current draw on the battery compared to its physical size and normal usage profile. We are asking it to deliver 1.5 to 3 amps, which is a heavy load for these small cells, which are normally asked to supply 250 milliamps or something like that. As a result the battery is simply too small to deliver this high current to a heavy load, and the result is that there is a voltage drop. The voltage drops from say 4 volts at rest to 3.6 volts under load. All batteries of every type do this, but the voltage drop for say a 2 amp load is in direct proportion to the battery's physical size.
This means a bigger battery is better as there will be less voltage drop under load.
When you add electronics into the mix, things get complicated. Simple regulator circuits will still suffer from the voltage drop effect as they cannot compensate for it - basically you have to turn them up higher. This why for example 5 volt regulators often found to deliver 4.8 volts when measured properly: they were tested and set for low loads that did not push the battery. Better regulators can eliminate the voltage drop effect as they have a feedback component that maintains a steady output voltage.
Ecig use involves a continual compromise between size and efficiency. It's just not possible to have a small ecig of any type that delivers a good result for an extended period of time. The factors: size - performance - time, are inter-related and all depend fundamentally on battery size. This is why some people like an APV with a 26650 cell in it - there are reasons for this. Equally, you may prefer a small one with a 14650 cell (which is close to the minimum practical size for an ecig battery, IMO), but you just have to be aware there are negatives to it.
Small size / performance / time: pick any two.
