*Slim doesn't mean healthy.
You can be a slim but if you are eating nothing but low fat "healthy" whole grains and other heavily processed carbs, that's not healthy. 2000 calories of junk a day isn't the same as 2000 calories of natural leafy greens, veg and unprocessed meats, eggs and seafood.
In the long term, the more natural diet is going to mean less illness, fewer medical conditions. It not about the calories.*
This is a very common misconception from the obese. 2000 calories a day is 2000 calories a day, whether it comes from healthy eating, sugar, fat, whatever.
Being obese is the biggest threat to health there is. I would be inclined to agree that between 2 individuals with a BMI of 23ish, earing a healthy diet would be better that one based solely on junk.
I would disagree with the suggestion that an obese individual eating healthy foods would overall enjoy better health than a slim one who eats junk.
Perhaps this too is part of the problem, people playing down just how much of a threat obesity is to health?