with the obesity epidemic calorie density should be the least of our worries.

Even if you're obese you still need calories from the base of your diet. Put another way, you aren't going to live on lettuce and kale alone.

if you ate lettuce and kale as much as an obese person eats chips, you'd be very healthy.

Um... I'm going to treat this as though you're serious. You're probably wrong here. Lettuce especially is pretty lacking in nutrients, and all of the good stuff in kale is locked inside thick cell walls that have to be broken mechanically or by heat. Moreover, you're assuming that someone is fat specifically because of chips, when they probably make a number of unhealthy choices that cumulatively lead to obesity. I'm not sure a 1 to 1 swap of kale for chips buys you much more than a sore jaw and very regular bowels.

I predict that in the near future the overlap of psychoneuroimmunology and gut flora is going to have a lot to say about regular, well-formed, easy to pass, mid to dark-brown, sinking, bowel movements.