"Not everyone had big families. My very working class parents born in the 1920s were 1 of 4 children, and 1 of 3 children, no sibling deaths."
Of course not, I don't think that anybody is saying that though. There was a rapid fall in household size in the UK after the First World War but that was a continuation of an ongoing trend since the 1880s.
Up until around 1880, the median number of children that a woman would give birth to was around five. This gradually fell to about 2.4 by the 1920s.
So, even in the 1920s, your parents came from larger than average families.
But there certainly were families with more children at that time.
I guess that I'm about half a generation younger than you, my parents were born in the late 1930s. My dad was one of six children as was my mum (although one of her siblings died at a young age).
A generation earlier than that, my grandparents (born between 1906 and 1914) one was one of nine children, one was one of seven children and two were each one of six children. My dad had 13 sets of aunts and uncles and my mum had 11 sets of aunts of uncles