There are 2 ways of writing ki in hindi . One is with (badi ee ki maatra) and other is with (choti ee ki maatra ) .Which one should be used when and why ?

Badi ee ki maatra is used when the sound of ee is stretched long. For example- paani, here we used badi ee ki maatra as the ni in paani is stretched. When the sound isn't stretched and is little contracted we use choti ee ki maatra. For example- diya, here we used choti ee ki maatra as di is contracted.