One problem with calories is it definitely is not a simple mathematical "calories in / calories out" formula. Roughly it is a good guideline. But the body also is able to adapt to changes in calorie consumption and mass. (See myth #1.)

http://www.nejm.org/doi/full/10.1056/NEJMsa1208051#t=article...

Most advice I've heard suggests that going way below your basal metabolic rate calorie consumption isn't that great of an idea, because your body adapts to that reduced calorie range (some people call this "starvation mode"). I would hazard a guess (though this is just a postulate without any data I could find) that this especially applies when you are at a more "normal" weight already.

There's a reason why weight loss is often described as a "lifestyle change".