You know, I don't have proof or a witty reply in store, all i know is that is what happens to me.
concerning the abs.
when i squat it just makes my abs blocky, I think its becuase I tend to expand my abdominalls on this lift,
on deads I have a predisposition in contracteing my abs, almost as if I am doing a vacume pose, so my abs may not physically get smaller, been it seems enough like it for me to notice
also I a m fairly tall so the motion of deadlifting lends itself to this effect.....at least for me.