Unless they added bunnies in the last update, you're clearly playing a mod and might not have implemented the litter size properly
? Dude litter sizes literally are not species adjacent at all and never were, mods or not, there's no relation.
I don't know, when I do it with a human or horse they usually have just one baby but when I do it with a cat or dog they have 3-6 or even more
It depends on the fertility and breeder perks you have active, as well as RNG from the amount of creampies you give/recieve.
makes sense but also looked into the code and noticed what looked like a matrix for how many children each species should have (but please note I didn't take a deep look)
Its fine to look. Or even make mods! Viait the discord fod more information