Not quite on topic (I agree that random gen/premades in TS4 tend to be oddly proportioned in comparison to TS3, I think it's just due to there being more customizability - however, I think the largest frame in TS3 is larger than TS4) BUT
The feminine frames in TS4 seem to have very low hips. It's difficult to explain but the widest point on the hips is stuck to a point very low on the torso. I'm sure this is a possible body shape and many have it IRL, but amongst myself and the people around me I feel like the hips tend to be a bit higher in comparison to the torso/legs. TS2 and TS3 seemed to be more proportioned that way. I wish we could have more customization for the waist/hips/legs, because for all the baseline customization added it's still very difficult for me to make an accurate simself.
Edit: looping back to the topic, proportions play a huge part in how thin/thick the frame looks. the exaggerated " ) ( " of most sims gives them the thinner look.
https://i.ibb.co/C7bLPxd/Screen-Shot-2021-11-08-at-3-15-04-PM.png
