There’s lots of discussion lately about weight gain causing an increase in fat cell numbers. I firmly believe the body will not create extra fat cells unless there's a direct need for it. Our body always seems to adapt to changes depending on its own available resources and needs. Simply gaining weight is not necessarily a need for new storage cells. For example it would be like ordering ten new buckets to store water when the first ten buckets are only half full. There is still space for water in the first buckets, thus no need to use more buckets. If all fat cells become full through obesity, then there would be a need for the body to create new cells for additional storage.
Forming new fat cells will probably depend on several factors:
Present number of fat cells
Present fat percentage
Activity levels – if you’re not active, or you have a sedentary lifestyle, the body may have a greater need to create extra fat cells
New fat cells or bigger old cells when gaining weight
I believe the human body will create extra fat cells depending on the need of the body at the time, and will probably depend on the physiological and nutritional state of the person. If someone is only 5 foot 5 tall and only has 100 pounds of total lean mass, then piling on an extra 5 pounds may be a huge gain for this individual, and if the present fat cells struggle with this storage it may encourage the body to develop new fat cells to accommodate extra storage. Yet for some people, this small, light individual may seem like they would never suffer with a weight problem. If however, another person also gains 5 pounds, but he is a 6 foot beefcake with plenty of "empty" fat cells, maybe due to losing weight previously, then this 5 pound gain may be easy for him to accommodate the extra fat without the body forming any new fat cells. I believe most people who gain lots of weight, already have plenty of space available in old fat cells without the need to create new cells. It could be the case that additional fat cells are only created if a person becomes seriously obese.
Genetically determined Fat cells numbers and losing weight
A look at some causes of Negative Body Image