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> My brother is just a 6' and considerably smaller than I am.

That is already very large, please don't interpret this as a sign that your brother is less healthy than you. If you are considerably larger than 182cm, then by some measures you'd be viewed as being less healthy than your siblings because you're prone to all the problems large people have.

It is very likely you all three maxed out the size range available to you genetically and epigenetically by receiving adequate nourishment, as is common in industrial nations.

> Might there be a biological imperative to ensure that the first born is healthy

Probably not as such, but the psychological effect you're describing might create that impression: first-time parents obsess about everything because they don't want to make any mistakes. With subsequent children they realize they've been overthinking everything and they consciously apply what they learned the first time, instead of indiscriminately following through on every little thing that comes to their minds (like giving up soda).

It's also a very first-world way of thinking about children.

> Then with the later children parent lose the need to ensure their children are as healthy.

Again, I think you're postulating purpose where none exists, because you see a correlation between your "better" health and the amount of attention your mom applied to arbitrary details when she was carrying you to term.

Instead, evidence suggests that increased survival pressure in humans leads to an increase in the amount of children a couple has, and attention to individual children (including first born) goes down drastically.

My own anecdotal impressions of wealthy industrial societies imply that while first-time parents sweat arbitrary details, it's really the last child a couple has that receives special attention. Partly because the parents have better skills, but probably also because they know it's the last chance "to do everything right".

Children are like making pancakes. You totally ruin the first one; the 2nd is almost right; 3rd one - perfect.

I think this is interesting, so I am sharing my story. I am the first born son, and I have two younger brothers, 2.5 and 7 years apart respectively. My mother said she took extra care in what she ate when she was pregnant with me, but she didn't give up any of the junk food stuff. Soda, coke, fast food like McDonald's, cake, etc.

When she was pregnant with the second one. Realizing she might have made some mistakes with her diet, she cut out soda and coke from her diet and ate less fast food. Especially since she wanted a daughter this time.

With the youngest, she worked part time as well. It's also during that time we moved, and she didn't have much time to watch she ate either.

There is not much size difference between me and the older younger brother. He is taller and leaner than me though. I am 6'1 and 205 lbs, and he is 6'2 and 180 lbs.

The youngest one is supposedly still growing. (Technically males stop at 25 right?) But he has always been smaller comparing to us when we were the same age. He is 5'10 and 185 lbs currently.

I think prenatal definitely play a role in a child's development and later life. And it's not always the case that the first born gets the best.

I don't think this is a biological imperative at all, rather I think it's just that experienced parents worry less. The first child teaches them that they can mess up a bit and the kid will turn out just fine. The second kid teaches that even more. Each successive kid gets less obsessive care not for biological reasons, but because the parents have become more confident in their parenting and more aware of how indestructible little kids can be (obviously not literally, but humans are quite resilient).

It sounds like you are too large though. You will be susceptible to more problems than either of your siblings.

So you're more healthy than your siblings because you're larger? (like you said; they aren't malnourished.)

Size was a great heuristic for health for the entire history of the world up until maybe 50 years ago. If everyone's growth is stunted some to malnutrition then the biggest people are the least malnourished.

It doesn't hold as much these days but it still "feels" right and useful.

Like many statistical tools, the height charts doctors use are gaussian distributions and the inferences drawn from where an individual falls on that distribution do not boil down to "bigger === better."