Unlike food, nicotine is not a necessity. Also calorie intake alone doesn't determine weight gain or loss. The problem of obesity is much more complex.
Governments try to address this problem through education and regulation of food. There are drugs available now to help control obesity and they're very popular, so people obviously want to avoid the condition.
I don't know why you think people should have a right to take highly addictive drugs that result in premature death. Contrary to smoker's claims, cigarettes are pure addiction and provide no benefits whatsoever to the smoker.
They already prevent advertising the sorts of foods that contribute to obesity to children, and encourage you to drink less sugary drinks by applying tax to them (though unfortunately manufacturers have responded to this by reducing choice and adding artificial sweeteners instead of selling something at a higher price that can be enjoyed once every few weeks.
I don't think any of this is unreasonable in a country that picks up the tab through both subsidised dental care and completely free-at-point-of-use healthcare.
A legislation that isn't possible to enforce is not reasonable, no.
Banning cigarettes = easy to enforce.
Banning sugar in soft drinks = easy to enforce.
Limiting how many calories you can consume = how do you propose we do that? Do we even have the technology to track what someone eats? And do we carve out exceptions for athletes?
If there was a way to cap calories without surgically inserting trackers into everybody I'm sure you'd see a lot less opposition to your idea.
Whether something is possible to enforce seems like a sliding scale. We can totally imagine a world where a calorie cap is possible to enforce. In such a world, would it be reasonable?
There are good reasons to target smoking given how addictive and deadly it is. Nicotine is fairly unique in this regard.