Well, as far as i know, there are diferent weight between characters for real , for example whem i do Arc wall combos on Nero, the timming its preety diferent if is compared whit the timming i have to use against for example Kohaku or Ciel on a wall combo, same as some Satsuki's combos. ( im a Satsuki-Arc player ).
But we need to have in mind, that all characters have diferet hitboxes, maybe that "weight" stuff its just an illusion.
I think that the only way to figure this out is watching some replays frame x frame, a framedate display will be cool for this i think.
Cheers, sorry about my crappy english xD.