I'm the 7th of 9. People are usually horrified and curious about the number of siblings I have. A lot assume that my parents are die hard Christians who reject birth control but that is not the case at all. They just had a bunch of kids.
I think big families seem incompatible with social trends. Its more acceptable, necessary and expected of women to invest on their educations and have a career before having children and usually, when families are this big, it means that the mother was young when she began to have babies and did not have a career. Most women today who have experienced financial and professional success cannot understand or even bother to ask why someone would choose not to have a career. It's a matter of personal preference but having a career is more the "norm" today.
It's also less common for couples to have all the children they can. Now most people think about having the amount of children that they can support and live comfortably. Big families have more financial obligations and things to worry about in terms of food, education, housing and health, so people don't get why some people willingly expose themselves to financial disaster or at the very least to a life on a tight economy. When the majority desires a life with one or two children living in an above average situation, or with little financial concerns its hard for those to get why others don't mind the trouble.
There is more awareness and urgency about environmental impact of people on the planet, so people who worry about sustainability don't see how it is "responsible" or "intelligent" to have so many people running around. Child development theories also advice against big families and more parents are concerned about what the "experts" have to say about having a family. Attention, time and financial security for one or two children is what most psychologists and educators advice and a lot of couples interpret this as being "the only way to raise a good family is to have 2.5 children and no more".
The reasons are infinite, in short it just means that Western life style is now incompatible with big families. Those who have many children willingly separate themselves from the majority and demonstrate by having large families that their priorities and ideas are elsewhere. People just have a hard time understanding that just because a lot of people do things it doesn't make it "common sense" or "the only way".