So yeah, long story short, I now have a named giant sponge called "Scalyrifts" in the water who keeps communicating to me telepathically about his kills.
I was also able to do some actual research. No, being standing does not seem to help at all. In fact, I had a superior strength sperm whale man vampire bashing around both standing and lying kobolds with the platinum maul - these he knocked consistently a square or two back with his tail - for, well, an approximation of 20 hits in each pose. I was getting kind of impatient as I failed to score one single knockback.
I created an experimental weapon called a "whale cudgel" to see if big sized weapons SHOULD hit for more. This is just a mace that's been scaled to sperm whale man size, 360 times larger than the human version. Now with this thing you could send kobolds flying. And from bashing around the kobolds a while with it, it seemed like it knocked back more with good, probable hits (and up to 3 squares from the prone position when unconsious). That's just an impression though, I didn't even attempt numeric research. But even this one was a failure at sending the humans flying. In fact, no matter what I did, this whale adventurer could not knock back humans with his tail. It may have been the strength boost from reanimation that pushed it over the edge last time? That's why I switched to kobolds, and made sure the tail strike was comparable to compare maul efficiency.
However, I have to say about knockback...
this system is broken. Nevermind that the bronze colossus thing didn't work; that was expected since misc. fighting is not necessarily very refined (although hitting something with something that's made of metal and weighs dozens of times more than the thing itself SHOULD do more than bruise). But in this latest test with the "whale cudgel", I am a freaking sperm whale man vampire, of "superior" strength compared to even my own species, 360 times larger than the human standing in front of me, and I swing at him with a mace that's a LOT heavier than him, being 4 times larger than he is and made of iron, and he still does not even BUDGE. That thing did cause deep tissue injury every time, on the positive side.
Edit: To give a comparison of what these characters are otherwise capable of, I grabbed the human by the tongue and threw him. He landed literally on the other side of the arena map, diagonally. Another wasn't so lucky, he was thrown by the throat from the bottom left corner of the map, slammed into the round wall in the center after flying 48 squares by my calculations and blew to pieces.