Knew a guy whose second kid ended up being twins. Barely seen him this decade.

I thought 1 to 2 was a bigger leap myself. You went from being able to hand off the kid and take a break to handing off the baby and getting the toddler in return.

To use your "man-to-man vs. zone" analogy, going from one to two is like going from playing defense to playing both sides of the ball. Sure, going from two to three, more stuff gets missed, but the exhaustion level and overall work load didn't change that much (in my experience).

Then again, maybe it's more about the age of the elders. We had two in two years and then didn't add the third until the big kids were 6 and 8, so they had already gotten the hang of basic life skills like getting dressed for school, getting themselves breakfast, reading, etc. Maybe that's why 3 to 4 was easier for you and 2 to 3 was easier for me.