Why is the inside spring larger than others on the muncie 4sp. shifter?
Just curious?

Not sure I understand your question.

On a muncie shifter there is a detent spring that is in the center of the shifter? Why does the spring look more heavy duty on some muncie shifters and thinner in diameter on others?

I have seen 2 different style reverse detent assemblies on the Muncie shifter.
The early one uses a ring shaped spring with balls and a plunger
The later style has a coil spring on the outside