The whole healthy diet and exercise thing imho is more challenging for frum Yidden, women and men.
-Gyms are limited.
-Yoga may be considered avodah zara and thus not permitted.
-Wearing skirts also adds another challenge to working out (though yes I know that women do it despite the restrictions, but there is a big difference in range of movement compared to untznius attire)
-Shabbos is full of eating and restricts walking long distances
-Yom tovim also full of eating and lounging
-For men, prayer occupies time that could be spent at the gym
-For women, raising children and being in the home or at work limits time to take care of oneself
Yes people somehow manage. Yet there is added stress and hurdles to get through. Stress alone inhibits metabolic efficiency, adds weight to the torso, etc. Plus being so busy can mean less sleep. Sleep deprivation is another way to gain weight despite genuine efforts to be healthy.
Back in the day I did this 5am bootcamp while going to college full-time. I ran around doing bootcamp crazy routines with a yelling drill sergeant every weekday. I may have gained some muscle but lost more body fat when I stopped the program and just slept more instead. I replaced manic morning bootcamp with evening fast-paced yoga and it was a way better fit.
The whole, what you put into your body and what energy you burn = what you will weigh is a myth. There are many additional factors here.