Why is wire size so important?


It is important for carrying current from solar panels. If it is large enough, you can have the maximum energy. Also, it is very important to calculate the wire size before purchasing. If the wire is small for the current, then it can be overheated and cause some problems. 
Here is a good discussion of wire sizing that I think will also be helpful.

According to NEC you need to over rate by 125 %  of  the max amps that the array / inverter(s) can produce,  in addition to a wire size that provides less than a 2 % voltage drop, according to Enphase.