An imaging Newt is optimised for imaging. this usually means a larger secondary as that ensure a larger part of the field is fully illuminated to maximise the amount of light falling near the edges of the sensor. Visual scopes ( and I am assuming that the SW one is that) do not need to be so careful about some drop off in brightness as it usually quite hard to detect by eye and most users only concentrate on the centre anyway.
In addition if one is an f4 and the other f5, all other things being equal, the f4 will need a larger secondary.
Albert Highe in his book (see
http://www.bintel.com.au/Accessories...oductview.aspx ) gives quite a detailed analysis of this matter which is well worth a read, although it can get a little technical.
Cheers
Malcolm