Yeah, you may have a point there ... Whilst a family with 5 or 6 or more children might have a few children who find it harder to be listened to sometimes, it's not a given. And there is no guarantee of being heard if you have just 1 or 2 siblings.
I just have 1 sibling, a brother, and he was definitely my mother's favourite child.. And even though my dad adored me, my brother was respected and listened to a lot more than me by him, and in the eyes of both of them, he could do no wrong.
My opinions were poo pooed as I was 'just a girl,' and I was taught to do 'girl things' like ironing and cooking and sewing and housework. I have always had an interest in science and astronomy, (and the planet,) but was never encouraged to pursue anything involving any of the above. Even the school refused to let me.
I had my wings clipped early on, and never developed the confidence to retrain/do things that interested me when I was older... Ergo, I am just in a bog standard admin job now. And THAT is only because I took courses at college myself in my early 20s, (in tech/office work/typing etc.) If I hadn't, I would have spent my working life in a factory, (where my parents got me to go to when I was 16.) Several relatives worked there, and they had no problem getting me a job on the assembly line! 
So I have a lack of confidence and I'm quite insecure, because I wasn't listened to as a child/younger person, and I am one of only two siblings. (Brother is 5 years younger...)
Thankfully my DC (now late 20s) are doing what they want with their life, as they weren't held back like many Gen X-ers!