1. Enlarged fat cells before weight loss, 2. Shrunken fat cells as a result of a caloric deficit.
When we exercise or diet, our bodies use stored calories as energy. If we consume less than our baseline calories (caloric deficit), we lose body fat.
Sadly, we do not actually lose fat cells (unless we engage in fat-freezing or liposuction), they just shrink in size.
Although fat freezing and liposuction can kill a portion fat cells, some stay alive. And the bad news is: when we diet and exercise, they simply shrink in size.
When we eat a surplus of calories, and do not exercise to burn them away, calories are stored as excess fat, expanding the fat cells.
Aside from diet (caloric deficit) and exercise, it has recently been acknowledged that cold exposure (cold thermogenesis) is a potent fat burner. In other words, it aids in shrinking fat cells.
Why? Because the body constantly strives to keep its temperature at 98.6 degrees F. When it's exposed to cold, it will do all it can to retain this temperature -- resulting in a very large caloric burn.