why is it abusive to have more than three children...
Even in a two parent household, parents simply do not have the time, energy, or emotional bandwidth to properly care for and manage that many children. This is irrespective of the space and money issue, which is also prevalent for many people.
Two things happen when you have too many children. It isn't always an either/or, it's sometimes both. One child, usually the oldest girl, is conscripted to raise the other children. Bath time, changing diapers, lunches, drop off and pickup at school, feeding, helping with homework, you name it, they'll be doing it. This isn't fair to the child who is forced to sacrifice their childhood to be a caregiver, but it also isn't fair to the child who is being left in the care of a minor who doesn't have any power. And/or one child, or more than one child, falls by the wayside.
Honestly, the strict defense of what "counts" as child abuse is that way to maintain a status quo that is in and of itself an abusive dynamic. But it's too hard for people to admit that their parents owed them better and were either unwilling to or incapable of giving it (in which case, they shouldn't have had children), and that being a parent is a responsibility most people half ass. And it's too hard for them to admit that they themselves owe their children better and are either unwilling to or incapable of giving it (and that they didn't have children because of any actual concern for a human being, but because society tells them they should) and that they themselves are half assing it.
Personally, I feel that most of the ways in which parents interact with their children is abusive, especially when it comes to the lack of control they feel children should have over the trajectory of their lives. I've point blank had someone say about her own children "I'm not gonna let some kid dictate my life" when I mentioned that moving cross country is a decision you should be making with your child and not for your child. At no point in that conversation did she consider or defend her child as a human being with wants and needs and feelings, but as a thing that she owned. It's true, you don't consult your couch when you have a second, third, or fourth child. You don't ask the fridge whether or not it wants to share a room with a dishwasher. That someone regards children in the same way is a bad thing.