Yep, BSFC uses terms of weight, so density and gallons does not come into play. I just put it in terms of hp/gph as the magic 20hp/gph is the benchmark for our little engines. Serious engineers stick with g/kWh.
20hp/gph is about 213g/kWh, the big beast is 171g/kWh. My little VW car at absolute best sweet spot on BSFC chart is 196g/kWh.
As engines get larger, it is easier to get better efficiency. As cylinder gets bigger, displacement increases faster than metal surface area, so a smaller fraction of the combustion heat is lost to heat transfer to relatively cooler metal. Flow losses less due to having more time to move gasses around.
Also, the design goals focus on longevity and efficiency, on little engines cost and performance are more primary.