Step 1: mobilisation. This is the process of getting the fat out of the fat cell.
Step 2: transport. This is the process of moving the fat from the blood into tissues that can burn it (like muscles, liver and the heart).
Step 3: oxidation. This is the actual burning of the fat.
For people with different body fat levels, different steps are limiting further fat loss.
In the obese, the oxidation phase is problematic. They have plenty of fat floating around in their blood, but the cells have become damaged, so they can't oxidize fat very efficiently.
At the other end, for lean individuals, mobilisation is a problem. It is difficult to get fat out of the fat cell.
In the middle, there really are no problems.