The science is pretty clear that too many people are obese, that it isn't good for them. There is not 'big boned', there is not 'my metabolism', and there is a HUGE difference between not fitting a BMI by a few points and being off the damn scale. The only people BMI doesn't work for are people over 6' 7", or people who are legitimate athletes. If you are a typical person...sorry that BMI is correct...or you're missing a limb.
There may not be such a thing as "big boned", but people do have different builds. Some people are tall, skinny and perfectly healthy with a BMI that would classify them as underweight. I'm fat, but I also have decent muscle mass. If I were to lose every single gram of fat on my body (not exactly healthy), my BMI would still classify me as overweight.
BMI is a gross simplification of body shapes. It may work for people with average bodies, but for some (on both ends of the spectrum) it does more harm then good. There are far better methods of determining whether someone has too much fat, like measuring their body fat percentage.
And people do have different metabolic rates. You can estimate your basal metabolic rate with various formulas based on weight, height, lean body mass and age, but there's still a large non-explained factor that, as far as we know, just comes down to luck/genetics. Quoth Wikipedia:
For instance, one study reported an extreme case where two individuals with the same lean body mass of 43 kg had BMRs of 1075 kcal/day (4.5 MJ/day) and 1790 kcal/day (7.5 MJ/day). This difference of 715 kcal/day (67%) is equivalent to one of the individuals completing a 10 kilometer run every day.
The mere fact that people like to use their build and metabolism as simple excuses for their weight problems doesn't mean they're not actual, legitimate factors.