Esme is a much smaller build than the others, she takes after Chris. Isabelle I guess gets her height from her dad's side. Isla takes after Sarah and they put on weight more easily. Even when Sarah was younger and not overweight, she never had a super skinny look. I'm a similar build to her, same height and can imagine if I stopped all physical activity and ate like every day is my birthday, I'd put on weight rapidly too.
Chris managed to not be overweight for a long time even though he was still eating unhealthily and not doing a lot of exercise. He was a smaller build, not that tall. He went skating but not that much and much of the "skating" time was doing other things. Even when there he was filming others a lot of the time rather than doing anything himself. Combination of eating even more, even less exercise, stress and age caught up with him and look at him now. Esme takes after him in that she is the only one who actually looks to be on the shorter side, but she will dive in to the sweets and massive bars of chocolate with just as much enthusiasm as her mother, she eats it so fast it's all over her face, food down her clothes. She's certainly not making any conscious decisions about choosing healthier food than the others.