It is their business, IMO. They provide for their children, seem to have a happy home and a good relationship. They shop second hand, don't buy things until they can pay for them outright, home-school, etc. I believe their family size has everything to do with their religious beliefs, which is fine by me so long as they can afford the cost of raising them without assistance from the tax coffers. If the public is curious as to how a very large family can be managed, and the Duggars are willing to open their home, then so be it.
I believe their family built and owns some self-storage facilities and other modest investments. And they simply don't spend a lot of money. This is an old-fashioned/traditional family structure that works for these people and I really have to admire them--even though I would never choose it for myself! My grandparents grew up with this exact structure and super-sized families because they were farmers, and big families were normal. My grandparents all recall their childhoods with lots of siblings as being wonderful.
I tend to believe that once her kids are grown, she'll be advanced in age and there will be so many grandkids running around that she will never be lonely. Plus, she and her husband seem genuinely fond of one another (unlike Jon & Kate...)
