bsfc is directly (edit:inversely) dependent on how thermally efficient a motor is. motor size only plays a very small role, if any.
edit: compare the 'average' graphs of the 40cc to the 91.6cc motors. the bsfc's used were exactly the same for each graph. (on the graph, bsfc is the lowest line).
somewhere on here in another thread, made a chart that shows how therm eff each motor is. there you can see what i mean.
edit: found the link
http://chainsawrepair.createaforum.com/ported-saws/saw-motor-data-technical-chart-(things-mfg's-don't-tell-you)/?message=44955what's interesting to me however, is the effect of fuel on the small vs. big motor.