How Is This Possible?

Because... It's a game? I get what you are saying. Yet if you want her to get "less skinny" then use cas.fulleditmode
I know it's a 'cheat code', but personally it's excellent to modify Sims. Doesn't have to be much. Just put the slider bar up from 'skinny' to 'fat' a little bit. Much simpler than the skin-tones. ;)

my sims had 8 kids and she does look like she gained some weight,

I imagine you're kind of poking fun, but yeah. The game doesn't calculate for baby weight.

I do know that if your sims feed the infant too often, the kid ages up to be heavier than their parents. I hired a nanny once, and only once, because my sims had twins. Well...she was constantly picking them up to feed them, even when they weren't hungry. Both of them were heavy set when they grew up, but their siblings were skinny. I let the babies cry for a little bit, waiting for them to need diaper change and feeding all at the same time. and the kids grow up skinny. But don't wait too long, they might get taken away. lol

I think its dependant on the preset body shape selection and fitness skill level. I have some sims who never gain weight after 6 kids, some will start putting weight on after baby #3, and some who gain a lot with 1!

"sarabeth2984;c-17932808" wrote:
I think its dependant on the preset body shape selection and fitness skill level. I have some sims who never gain weight after 6 kids, some will start putting weight on after baby #3, and some who gain a lot with 1!

I notice that if you start out with the sim on the skinny side, they almost never gain weight. But, if you start a sim in the center or on the heavy side, then it is very easy for them to gain weight. They also gain weight if they are skinny, but if you change their thickness. I usually make my cultural sims with wider hips and thighs and butt and they gain so much weight after having babies. Don't even get me started on my Pixar moms. They're huge now!

I usually don’t have lady sims with kids but when they do have them, well…..they sure look like they had a kid!

Most of them start off as a skinny little thing, but after having their baby they would have grown a lovely pair of hips and a lovely pair of … Anyway I do have both body presets and height slider mods, so perhaps this is the reason for the change? The game might not recognize her as “skinny”?
Sometimes I make the new mother go jogging for a couple of days, but usually I leave her the way she is. Sure it only looks healthy. ?

"sarabeth2984;c-17932808" wrote:
I think its dependant on the preset body shape selection and fitness skill level. I have some sims who never gain weight after 6 kids, some will start putting weight on after baby #3, and some who gain a lot with 1!

This has been my experience with it since launch. I'm still seeing a range of weight gain, varying by Sim, which is really awesome!

"Calico45;c-17932706" wrote:
I imagine you're kind of poking fun, but yeah. The game doesn't calculate for baby weight.

I was not poking fun, as you put it. Just my observations.
Without cheats or mods, or buying or making potions (same thing) Sims generally muscle up or down and skinny/fat up or down on their activities and how much or little they eat.

"Admiral8Q;c-17932712" wrote:

"Calico45;c-17932706" wrote:
I imagine you're kind of poking fun, but yeah. The game doesn't calculate for baby weight.

I was not poking fun, as you put it. Just my observations.
Without cheats or mods, or buying or making potions (same thing) Sims generally muscle up or down and skinny/fat up or down on their activities and how much or little they eat.

Good. Then saying it is a video game as the reasoning won't come off as sarcasm then, because honestly that is it. It was not programmed in.

You are technically right, but the Sims 4 calorie system leaves much to be desired. I dare say it has largely been ignored by the dev team. Anyway, there are guides to it developed by much more dedicated players than me. I just know most of the time I use DLC recipes and my Sims do not seem to gain an ounce.