Why is there a major and minor barrel in a pig launcher and receiver?

A major barrel is bigger than the normal piping in the system to make it easier to insert the pig and launch and receive it. The minor barrel has the same inside diameter as the main pipeline and is used to clean the pipes.

Pig Launchers And Receivers With Safety Locking System ( 675 Psig At 200°F)
Pig Launchers and Receivers with Safety Locking System
( 675 psig at 200°F)