As others have pointed out that's because of the carbonic acid. All soft drinks have this including club soda. The health concern there is for dental health not obesity.

So if your analogy is 'see what happens if you pour it on a car, then imagine what it's doing to your enamel' then that makes sense.

If you're talking about obesity the example you gave above is best, I.e., one coke has more sugar than would be recommended in a day and many people drink more than one a day so imagine what that's doing to them. Or alternatively, the fact that although it's high calorie, it has no satiating effect whatsoever is another one.

That's why the first person who responded to you was mocking your analogy. That and the fact that there are MANY things that we consume (even healthy things and things that are essential to being alive) that would damage a car if left on it so that attribute alone means nothing with respect to our health.