Based on the Fiat 500, the Fat is designed as a compact city car similar in size as the Bug. The name "Fat" is simply being "Fiat" with the "i" removed.
Grand Theft Auto: London
Like the Bug, the car is considered a low-end vehicle in the game, although it features average performance. Due presumably to its lighter weight, the car's underwhelming engine is able to perform better, resulting an average top speed, excellent acceleration (the best alongside those of the Myni and Voxel Concept) and good handling. Its grip, however, is below average, and the amount of damage that the car can soak up is 20% less than the vast majority of vehicles in the games (similar to the Myni, suggesting size is a factor).
Grand Theft Auto: London 1969
The base value of the car when imported is low: £200 in mint condition.