No problem here with .250, chatter or programming .... but I see what you are meaning.
Russ

O.K. guys, I checked, my version is 1.0.13. It seems my radius request of .250 was causing the width problem. The .250 radius is what I need so as to have perfectly rounded ends to the rectangular pocket not squared (inside pocket). So I tried a .125 cutter and asked for a .250 radius and same problem. I really don't like straight down plunging. I'm open for suggestions.

See if the Keyway slot does a better job for you. It does have fully rounded ends, and it will do a .500 wide with a .250 tool. It uses a ramp in start on a straight line down the center of the slot.