The fence is either too long, uses poor wire, or an improper grounding system. The charger output decreases to the point that it won't control the animal. The fence is flawed in a way that impedes the output of the charger. On high Resistance fences, it takes a much more powerful charger to deliver same output a smaller charger could on a well designed fence the same length.

The fence uses good wire, is properly grounded and utilizes jumpers. You will get the maximium output from the charger, and the animals will too! A smaller charger on a good fence can outperform a more powerful charger on a poor fence any day.