The cable size is actually nominal only for naming purposes. Usually it will be larger than the specified name. This is because there's many different grades of copper, a low grade one will have more resistance which gives more heat and will melt the insulation.

As per IEC 60228 and MS2112 cable size is defined by The maximum permissible resistance per unit length (in ohms per kilometre – Ω/km) of each conductor size, class and type

https://en.m.wikipedia.org/wiki/IEC_60228

To make things simple, just make sure ada sirim