Checked FAA Advisory Circular 43-13.2, Chapter 8. May or may not help Euro ops, but...
Early on it explains how the structural limits of various tow aircraft are determined. There is a simple formula given for that determination. Tow limits for US aircraft are based on structural limits for the aircraft, not performance limits. Generally, the FAA uses the breaking strength of the tow rope to determine the max weight of the towed glider. Also that weight is usually placarded around 1200 lbs. maximum towline strength to comply with the structural limitations, depending on the aircraft.

In conjunction with that, the FAA also states operationally that the tow line strength for the glider must be no less the 80% of the glider weight or no more than 200% of that weight. So, at least in the US, a tug with a 1200 lb, tow line limit can legally and theoretically to up to a 1500 lb glider (Cub and 2-32?), and still be with in the limits of the towing aircraft. The aircraft performance capabilities are another story and are the responsibility of the operator.

Don't know if this could help or not.

Linwood