I don't think there's a link to large families and emotional abuse to the eldest child. I would rather link toxic parenting to abuse of the eldest or even the only child.
My mom grew up in a family of 12 and the eldest child was not emotionally abused in any way. Instead, the family is and continues to be extremely close, even if half of them have passed away. I don't know what the secret is to such support they give each other, but they are always there for one another and for all nephews, nieces and grandchildren.
I come from a family of six children, and I can't link family size to emotional abuse to any child. In fact, I loved holidays because there were so many of us and we always had a party even if it's just among ourselves.
We have one child, and holidays are rather quietly celebrated by contrast. Her EQ is rather good, I must say.