This is related to the short-circuit performance of the cable - the minimum size to withstand short-circuit current. Cables must be sized to carry the load current and withstand the fault current should a short-circuit occur. 

In Cable Pro Web software there is an input for Fault level (kA). This input will be used to determine the minimum size required to withstand short-circuits.

When the load current of a circuit is quite low and the length of the cable run is quite short then the minimum active size may be dictated by the fault level.

To fix the problem - reduce the Fault level (kA) input to a reasonable value.

See the two images below for example.

Image 1 - Fault level 5 kA, 10 m length; Active size = 1 mm2

Image 2 - Fault level 5 kA, 5 m length; Active size = 6 mm2