Some doctors now think that the internal fat surrounding vital organs like the heart, liver or pancreas — invisible to the naked eye — could be as dangerous as the more obvious external fat that bulges underneath the skin.
...
“Being thin doesn’t automatically mean you’re not fat,” said Dr. Jimmy Bell, a professor of molecular imaging at Imperial College, London. Since 1994, Bell and his team have scanned nearly 800 people with MRI machines to create “fat maps” showing where people store fat.
...
According to the data, people who maintain their weight through diet rather than exercise are likely to have major deposits of internal fat, even if they are otherwise slim. “The whole concept of being fat needs to be redefined,” said Bell, whose research is funded by Britain’s Medical Research Council.
...
Even people with normal Body Mass Index scores — a standard obesity measure that divides your weight by the square of your height — can have surprising levels of fat deposits inside.
...
Still, most experts believe that being of normal weight is an indicator of good health, and that BMI is a reliable measurement.
...
The good news is that internal fat can be easily burned off through exercise or even by improving your diet. “Even if you don’t see it on your bathroom scale, caloric restriction and physical exercise have an aggressive effect on visceral fat,” said Dr. Bob Ross, an obesity expert at Queen’s University in Canada.

The caveat for BMI is that it's a reliable measurement for most people but not for those with a lot of muscle. Sometimes a higher BMI is the result of muscle mass figuring for a larger portion of overall body composition.

The generic scale for BMI for males usually considers 26 to be the cutoff from being fit and being "overweight." The study described below found 26.5 to be the "ideal" for males. How can it be ideal to be overweight? which makes the value label of "fit" and "overweight" a bit arbitrary.

World's most beautiful couple: and the figures to prove it

It is the holy grail of the fashion and beauty industries: a scientific blueprint for the most beautiful women, and men, in the world.
Researchers have thrown away the old vital statistics and, instead, focused on how the dimensions of different parts of the body relate to height and body mass index (BMI) to give the perfect physique. Perhaps surprisingly, two of the most important measurements are the girth of the thigh and the slimness of the calf.
Perfect woman: Naomi Campbell

The vital statistics:

Body mass index 20.85

Bust girth to height 49.3%

Waist-chest ratio 1.4

Leg-to-body ratio 1.4

Calf girth to height 19.5%

Height 175cm

Thigh girth to height 29.7

What it all means:

"Super beautiful" women have waists a third smaller than their hips and three-quarters their bust measurement. They have longer legs, and slimmer thighs and calves than the average woman.
Perfect man: Christian Bale

The vital statistics:

Body mass index 26.5

Waist-chest ratio 0.6

Leg-to-body ratio 1

Height 188cm

What it all means:

The physically ideal man is more than 6ft tall, with legs the same length as his upper body. The leg-to-body ratio of 1 makes him appear more muscular, which is why the ideal BMI for men is higher than for women.