choice of large diff cap and small cap

I can not find if anyone has the relative post or not.
I want to figure out when to choose large diff cap and when to choose small cap?
Is it relative to the size of the spur gear or the brand of truck?

[RC-Monster Mike]

The large endcap is for the slipper side and the small endcap is for the other side of the diff cup - one of each is used per unit. :)

Thank you. Mike.