Obesity in humans is not dependant on the body weight but rather on the quantity of body fat. Body fat is nothing more than an extremely loose connective tissue made up of adiposities. Its’ main function is storing energy for our body. There are two forms of adipose tissues:

Most often, body fat is measured as the percentage of fat in your body. For example, if you have 150 lbs. of weight and 30% body fat, then you have 45 lbs. of fat and 105 lbs. of lean body mass. Lean body mass consists of bone, muscle, organs and anything that isn't made out of fat. 

Fatty tissues are normally located underneath the skin but can also however surround the internal organs in order to provide sufficient padding. Body fat, however, can also act as a big reserve of bodily nutrients. 

A new born child has about 5-6 billion fat cells, and as he matures this number increases to about 25-35 billions fat cells. When fat cells increase in number, it's called "hyperplasia". When a fat cell increases in size, it's called "hypertrophy". A person becomes overweight because of extreme “hypertrophy”.