I think Kiyoshi's parents had so many children for several different reasons. The first one that come's to my mind is that birth control was not an option back in that time. Some forms of birth control were known by some cultures, but it wasn't a thing that was practiced in those days. The idea was men and women got married and had a family, and you were blessed with as many children as you had. Nobody thought about numbers of children. They just happened, and were accepted. In these modern days, some countries actually regulate how many children can be born to each household, and in other countries, people are concerned about the impact that many children will have on them.
I think another reason for so many children might be because they thought more children would bring greater fortune and happiness to their family. Japanese culture is filled with many beliefs and superstitions, and having many kids might be a part of it. There is always the belief of the First Born Son, who is to take responsibility for the family in the event that the father is gone, but if the First Born Son is unavailable, then that position goes to the Next Born Son, and down the line. Having many sons was more ideal than daughters, whom had little value to the family. But ideally, many children could also bring many more connections to other families and wealth and opportunity from their future marriages.
Personally, I came from a large family, with many siblings, and I wouldn't trade it for all the money in the world. We have wonderful memories of growing up together, the good times definitely outshine the bad.
