Incursion,
I'm gonna try and share the GREAT demo I got from my local gunshop when I asked the same question. Yeah, you already got the "definition" to your question, but I like the visual aid.
Get a tape measure and a piece of paper. Mark 2 lines on the paper, one inch apart. Now, extend the tape measure out 4". Hold the end of the tape over the paper. Imagine that the spool end, the one in your hand, is a turret, fixed in place. Touch the left line, with the end of your tape measure. Now, still keeping your hand in the same place, move the end of the tape measure to the right line. Extend the tape out to say... 11", and do it again. You will see that a change in 1" at the "barrel" end of your "gun" with a 4" sight radius will effect your point of impact SIGNIFICANTLy more than the same shift of 1" at the "barrel" end of your "gun" with an 11" sight radius. This is why sight radius effects accuracy.