The actual size of the fire is smaller..but much more efficient use of BTU output in a pellet grill. Just look at the size of the typical pellet grill burner. Total volume ( which is NEVER full...) is probably 2 cups of pellets? But - again you will never see one with more than 1 cup of smoldering pellets in it.
But, because the fan is controlled by a temperature probe, feeding this data to a microprocessor "controller" to adjust the needed amount of BTU's. Much more efficiently than we could do using splits by hand. For a number of reasons. Variablity in wood itself too dry? Not dry enough? Species? Growing condition/nutrition/cellular density unknowns?
One of the things that we do know about pellets is a pretty close BTU output based on oak, and some of the other woods have been tested.
The controller, fan and auger don't get sleepy or forgetful. If the temp is above or below the hysteresis or "dead band" - then the grill adjusts the rate of pellet feed, and or fan on/off or fan speed on variable speed models. They really cook like a wood fired convection oven - constantly moving the heated air around rather than relying on jus the convection/drafting of the chimney on a stick burner.
I know that the common belief is held that the thicker the steel...the better a grill is. That is only partially true. It is more durable perhaps. But, it does not "hold heat" much better. Steel is a great thermal conductor. We know this from the scars on body parts that have come in contact with a hot grill...
It radiates the temperature right thru it. Steel is not a window or door that can be closed and contain heat. It is more like a screen door. Yes, it will slow it down a little....But the thicker the steel - the MORE BTUs required to "load" or pre-heat the steel of the cooker itself in order to get it up to the decide heat. AND....the more BTUs required to maintain the desired level of heat.