Newly Aged Sims loose weight when I first open CAS

Hello there!
I now had this problem the forth time with my Let's Play Family. When I age my Sims up from Child to Teen, they are far to slim in CAS when I open that. And they stay that slim when I go back to playing. In the Picture you can see the problem. I had this with all of my boys and from child to teen. When my girl aged up, everything was fine. But the difference other than gender is that she aged up in Granite Falls. All the boys aged up in Willow Creek.
First I thought that was normal, but they are so slim that I am not so sure about that anymore.. And before I opened CAS they where all normal weight.

Did someone come across a similar problem? Google did not find me anything about that..

Thanks a lot and happy Simming!
LilyLadybug

The only problem with that is, that I enter CAS right after they aged up. So they would not have any time to put on some weight, would they?
And with the Sim on the photo you can see, that he had some muscles when he aged up, which would be correct, because he was a level 10 in the child Athletic-Skill. But nothing of that is left in and after CAS...