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The smaller one is for the stamped steel/welded arm, and the bigger one is for the forged arm

 

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Copy/pasted from an Ebay guide:

1995 - GM introduced a new lower control arm that was supposed to make the ball joint replacement easier. This new lower control arm is the stamped/welded design that is found on most K1500 and K2500 models through 1999. They still were producing the old control arm with riveted ball joints until mid 1995 so both designs are out there for the 1995 models. The control arm has a forged insert at the end where the ball joint is mounted, this ball joint hole diameter is 45.79 mm

1996-1999 GM introduced the Forged lower control arm that was used on W/T trucks and most one ton and diesel models. This arm also has a press-in ball joint and the hole diameter is 47.89mm