Well in her 100 baby challenge she did start the garden salad thing and she tried to do the jogging like you said but I think she ended up focusing on just getting the challenge finished. She wasn’t playing with mods back then either. Obviously if she used a very slimmed down Chelsea now, people would instantly call her out in the comments. I only really looked into it because I’ve played with a lot of plus size sims so I know how their genetics work and I was curious.Honestly, I would not put it past Kelsey to alter the weight of her "sweet baby angels" and edit it out. I know I had a matriarch that somehow got that healthy lifestyle trait and cooked from that menu but when she had twins with Father Winter, they still were on the curvy side of things. I am amazed that she even let Chelsea get to that weight. If I remember correctly in the original 100 Baby Challenge, when Chelsea started gaining weight, Kelsey got her on a whole jogging routine to make her lose it.
With the pics I posted you can see that the sims I’ve grouped together have exactly the same amount of muscle and fat which might make sense with the same 2 parents but with 1 plus size parent and a whole host of different sized sims (Vlad, Father Winter) it’s very strange they would be the same size.