While it's true for typical cases at a micro level, people do "choose" to get fat little by little with every dietary choice they make, there's a lot of ways that it ends up out of your control:
- your parents overfed you while you were a child and now you're fat before you really had agency
- you had a medical condition and your medication is associated with weight gain
- you have a stressful lifestyle and have fallen on eating as a stress relief mechanism, leading you to gain weight
- the food you have available in your neighborhood is all junk and you get fat as a result
- the cuisine in your country is generally junk and portion sizes are out of control (I'm looking at you, America)
- everyone in your community is fat and so you consider that normal
- you do just have slower metabolism than others and so staying lean is just more effort
Not to say that it's outside one's control to control their weight (I live in Japan, practically nobody here is fat), but it's not quite as simple as choosing to smoke or not. Lots of ways you can end up fat "by default" whereas smoking does involve the explicit choice to start (by default you don't smoke).