Having just read the wikipedia page I linked above, maybe that is not the best place to get an answer to your question. As far as I know the Schmidt-Newt is like a normal Newtonian, but it has a corrector plate like the SCT which allows the use of a spherical instead of a paraboloidal primary mirror. As opposed to the SCT, in the S-Newt, the secondary does not reflect the light back towards the primary but out the side of the tube like in a normal Newt, and it is not used to increase the effective focal length of the scope, so it is not oversized = more contrast... but half of what I'm saying here is a guess...
