What you're suggesting is in direct violation of the rules of physics. Given two pellets at equal velocity, the heavier one will penetrate more, despite it's larger size.
Period.
Secondly, are you saying that smaller shot doesn't drag feathers into wound channels?
Cause I've got dozens of grouse carcasses that would prove you wrong.
As a high school physics teacher I agree with you in principle. However the "rules of physics" only apply in lab conditions and really not even then (the "rules" really only apply in that magical world with no friction). With shot striking a bird there are so many variables in place that many of the
"rules" appear to be falsified or altered. They are not obviously but are simply being affected by variables that gun testers don't pay attention to.
Too many people ignore cross section when shooting steel shot. Smaller shot is less likely to strike a feather shaft and drag the same amount of feathers into a wound as is large shot. Smaller cross-section also has better initial penetration due to less frictional resistance.
After doing some labs with regards to inertia (firing different masses of the same size and diameter into styrofoam and measuring penetration --of course denser penetrated further). I tried out my bone breaking theory.
Using aluminum rods with rounded ends -- one with a diameter twice that of the other (so density was the same). I fired both with compressed air at the same velocity at the same diameter hard plastic pipe. Both shattered the pipe when fired straight on, however the smaller diameter rod was more forgiving at angles, breaking the pipe at more obtuse angles than the larger diameter one.
Tried penetration tests too. Fired straight into styrofoam, the larger diameter rod (therefore the heavier one) penetrated better but when I laid down layers of plastic flagging tape in front of it, the larger ones dragged more of the tape into the styrofoam and penetration was severely reduced, while the penetration of the smaller rods was virtually unaffected.
Again, if your happy with what you use, so be it. But I still contend that inside 30 yards that 6 steel with high velocities 1400 fps+ is a better killer on ducks than 4's and up. Beyond that larger sizes may kill more but I'm willing to bet that a lot more "misses" are actually heart-lungers with a dead bird piling up a few hundred yards away.