Hm, well, we have cupcakes maybe once a month at our gym. Nothing even remotely close to what people are stating. So I guess if it is that much issue, then maybe someone should say something. But we just don't have this problem. Smaller treats (like a small piece of candy) are more frequent. I don't see a problem with that. I don't believe in restriction of any kind, but I do think that primarily "good choices" should be offered and modeled. I don't eat a standard American diet. I also strongly agree that a lot of things that are marketed as healthy are NOT HEALTHY. And actually full of sugar.
The average pudding cup purchased in stores or even made with a powder has negligible amounts of processed dairy and a TON of sugar, probably as much as a cupcake in some cases even. Even yogurts marketed for children have an absurd amount of sugar and dyes. Honestly I would consider a homemade, dye and preservative free cupcake to be a better choice over those kinds of yogurts, especially in the long run since the body burden of all these synthetic foods we consume can also interrupt the endocrine system and cause all sorts of issues. This is likely a major factor in obesity. To be honest, I don't avoid most of this stuff (like dyes and preservatives and processed food), but I am sure it is not good for us and try to limit it. But I don't limit sugar like in homemade goods.
As far as "ballooning up" - well okay, that happens sometimes, but pretty much everyone I did gymnastics with is active still. I haven't gained any weight personally, and I eat a lot of sugar. Everyone is different and personally sugar doesn't affect me that greatly (I mean I'm sure at a certain point it would, just not at regular amounts), just like gluten doesn't affect me. I have cut them out - no magical difference. Simply my body processes these things fine in a balanced diet. Also, doing gymnastics when you're young isn't going to reset your genetics. There have for ages been "sturdy" people, I am not denying there is a problem within a certain range of like the top percentage of obesity, but the bottom line is some people are more solid than others and fitness is the most important thing. Being able to run, move around, and have upper body strength (this is huge since many women simply never gain ANY upper body strength, and it becomes harder and harder as they pass through puberty and beyond), at whatever size you are, is the key. In other words fitness of the muscles and cardiovascular system.
You could be at the lower end of BMI and have poor fitness measures and have problems. And I have seen women who are classed as "obese" that are actually really fit. At the top end of obesity are usually people who have been extremely sedentary for a long time, and then become disabled and essentially never moving again (without serious intervention, which doesn't always exist). At these extremely high weights you could theoretically lose like 10-20 lbs per day with the right interventions, but at that point the systems in the body have become so dysfunctional it is incredibly hard. The short version is, I think it's incredibly misguided to focus on food beyond offering a balanced diet and drinking water. THIS ONLY LEADS TO PROBLEMS. The focus should be on encouraging fitness and cardiovascular health.