First of all, do not make the dough very tight. If your dough becomes very tight unintentionally, just add a little yogurt, and oil into the batter. Very tight dough may be caused for very hard koftas.
There is also another trick. Add a little more baking soda into the mixture. This will help to make your kofta balls soft and the size will be bigger.
[* These tricks are applicable for any type of kofta.]