The required driving licence has everything to do with the Gross Vehicle Weight (GVW) of the towing vehicle + trailer. Not to be confused with empty weight. The GVW is defined by the manufacturer during homologation of the vehicle. 

With driving licence B you are allowed to drive with a trailer with a GVW up to 750 kg, or when the sum of the GVW of the trailer and the car does not exceed 3500 kg. Example: towing vehicle with 2000 kg GVW + trailer with 1500 kg GVW = 3500 kg, the limit for driving licence B.
With an additional training you may tow up to 4250 kg, so 750 more. If you get above this, a trailer driving licence (BE) is required.