The screws are indeed different between the large and small Sebenza. While not too noticeable from pictures there is a fair difference in size.
It's a guess on my part, but I would like to think that the large and small models were designed to have much the same thread bite percentages and therefore assembly strength.
On a similar note, one screw I always worked very carefully with when changing was the pocket clip one. It's direct into the titanium and I found that it was easy to destroy the screw-head with too much force. So it pays to give extra care and attention there.
As far as the fittings varying on different models of the same size, I have been looking at three large Sebenzas just now - a regular, classic and 21 and they seem to be the exact same. Of course, there may be minor differences but I think the fittings used are the same.
Cheers,
Mark