When we consume an excess amount of calories without utilizing them as an immediate fuel source or "burning them off", the body stores some in reserve in the form of gylcogen. Glycogen is mainly stored in hepatic and muscle tissue. The remaining calories are converted to adipose tissue or fat. Adipose tissue is distributed throughout the body and finds a resting place in all cells. Over time as we continue to intake excessive calories without utilizing them as a fuel source the initial adipose tissue cells become larger and the total number of these cells increase.
When one becomes "fat" the size of the adipose tissue cells increases and the number of them also increases. Increased consumption of calories and especially increased consumption of calories derived from fatty foods is a major predisposing factor for many chronic illnesses.