Perhaps it is that they feel sorry for them - being in a large family, I guess, you never feel alone, and always have someone to turn to in crisis. If a parent dies, or both, you still have each other. If a parent or sibling is ill - you have someone else to take turns with looking after the parent or sibling.
However, only children never have to share their parent with another sibling, as well as many other benefits.
I am part of a very blended family which by it's nature makes me one of 8 girls (step and full siblings) - but I don't have any issue with people who are only children, or who only have the one child, or even plans for only the one. Difference is what makes the world go round.
I think as more and more people struggle with infertility and the cost of treatment for such - only children will no longer be 'odd'. Sometimes, thats all you are destined to have!
I think that with China's one child policy there is a slight issue - and it's not the "only child" thing. It's the disappearance of the "aunty" and "uncle" type people in a person's life. But, that can be worked around too - I have several Aunties and Uncles I'm not related to!