The level of chocolate toxicity depends on the type of chocolate the amount consumed and the size of the dog. Therefore eating a small piece of chocolate may not be as harmful as eating a half bar.
First different types of chocolate are toxic to varying degrees. Darker chocolate is generally more toxic. A dog is more dangerous if it consumes more than the recommended dose. Another factor to consider is the dogs size. A medium or large dog will be fine if it eats a small cube of white or milk chocolate. A small lap dog however may show signs of an upset stomach if they consume the same amount of chocolate.
Large or medium-sized dogs who eat very small amounts of chocolate may not require veterinary treatment they may not even show any signs of chocolate toxicity. However you cannot say the same for small dogs. A small amount of white chocolate rarely poisons dogs but a small amount of dark chocolate can be extremely toxic particularly for small dogs.