You have to remember that once the tug/glider combination has accelerated to climb speed, the only force the tug has to overcome is the drag of the glider. A glider weighing 400kg with a L/D ratio of 1:40 will only provide 10kg of drag to the combination, plus a little bit added for the towrope. This is why even a fairly low powered tug such as a Chipmunk with a Gypsy Major and the standard Fairey Reed prop will still climb at close to 500fpm with a standard class glider on the back.
Getting the combination up to flying speed is a different matter though.