Muscles.. :D

I started playing my quite thin sim.. She did not do any work out.. she is painter And yet I managet to get "body-builder" figure I have been fishing for a moment.. could that be the reason ?
Its funny.. yet...SAD I want my thin sim back

Pretty much you have to do cardio workouts(POSSIBLY, lol, eat more to get rid of definition), that is meant for Sims to get thinner in Sims 4. They REALLY want everyone to put effort into their Sims figures.
Here is a cheat I found, it will let you modify your Sims body. I have not experimented to see if it does changes, cause I went in there once(I went in there earlier today I mean, Lol) to see if my Sims hips were maxed still and they were, Lol. So I didn't do a COMPLETE experiment, but the morphing was in effect though when I entered, Lol.
Cheat:
Press and hold, Ctrl, shift, C, at the same time, for cheat box.
Type in, testingcheats.(without the period at the end)
When you click enter you will see it says it is "enabled"
Then type in, cas.fulleditmode.(without period at the end again, Lol)
Click enter, it won't say anything like enabled and stuff.
Hold the Shift Key, click on your sim, and click "modify in CAS" or click the button that has the word just "modify" with in it.
An loading screen, and you will see your Sim, test it by Highlighting your Sim a.k.a just hover your cursor(mouse) over your Sims body parts and it should high light the EXACT SAME way like in CAS.
You HAVE to type all that in and do that so you can modify body parts if you don't type the second thing, it will be like "plan outfit" and "children lifespan"(when you are creating a child in CAS) mode, where you can't do JACK! Lol. Hope this helps you and that it shows the changes when you leave the CAS cheat mode, cause again I couldn't see my results cause my sim was STILL the same and I myself was to tired to wait through FAST LIKE loading screens, mine take 22 seconds or so(normal pace counting, not fast counting), Lol. I Wish you Good luck. Also if this doesn't help just do the cardio workout and such, Lol.

I am going to try it right now I wasnt able to dream properly in my sleep All Icould dream about was muscles

Its working..thank you

Awesome glad I could help you.