HELP - Anamoly in Stress concentration

bloodlustdk

I am joining two segments of a boom with a shank and ran the stress analysis on it. The shank diameter was the same as the hole in boom. I got the peak stress of 100 mil. Then I reduced the shank diameter of the shank by .5 mm and the peak stress dropped to 50 mil. This seems counter intuitive to me. Can anyone explain to me why this occurred? I ran the analysis in Solidworks.