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'll send the design files if this description is not clear enough.

[JD Mather]

What contacts did you use in each analysis?
Attach your assembly here.