There are two major factors (related) that will affect the efficiency and capability of a heating system - including a boiler. One is the actual combustion of the fuel......you must burn it efficiently in the first place! With a pellet Central Heating
system, this is not a big problem since the fuel can be metered and burned hot in a relatively small fire pot.
The second part, which relates to the boiler construction, has to do with the available heat exchanger to properly soak up the heat from the burning Pellets
and transfer it to the water - and then to the house. This is where a Pellet boiler may differ greatly from a wood or coal hand-fed unit.
There needs to be a certain amount of square feet of heating surface which I think is defined as the surface of steel with water behind it that is exposed to the heat of the fire. In a case where you have a very controlled burn such as Pellets, this might be a series of tubes or baffles located above the pellet fire through which the gases pass. The engineering of the boiler should carefully match the heat generated by the fire to the heat which is able to be soaked up by the water. Excess heat exchange is not desirable - you only need to soak up the right about of heat to create the efficiency you are seeking - no more. Also, having a water jacket around all sides of a pellet boiler and the bottom (these are know as wet leg or wet bottom boilers ) is not as important as with wood or coal boilers, since the heat from the flame can be contained and directed into a small area.
Types of Boilers
Firetube - a boiler with round tubes through which the exhaust gases pass - these are either horizontal or vertical and surrounded by water.
Watertube - a boiler with horizontal tubes above the fire which have water circulating through them - the exhaust gases pass around the tubes.
One has the water on the outside of the tube and the other on the inside. I assume that in most cases a firetube boiler is the preferred heat exchanger type.
As a summary, while water volume and sq ft of heating surface may be important with a hand-fired wood or coal boiler, it would seem that a Pellet boiler could be engineered to use only an efficient heat exchanger on top of the fire and not a lot of water storage in the boiler itself.