The way I understand it that while there are many variables that contribute to a stroked small block gaining more power per cube than the respective big block. A part is power to weight,but the biggest factor is air speed through the heads that is gained when one goes for big strokes. The higher the air speed, generally the higher the volumetric effeciency of the motor, and thus more power per cube and at lower revs. You can get volumetric efficiencies of greater than 100% in good examples, this means in real terms is that the air is rushing in so fast that a little bit more is crammed in with the rush. It is no secret that very efficient smaller port volumes gain the most power, read Torque, per cube weight shifted.