Busbar size is very important as its directly connected to the electrode of the battery cells and if the busbar size is not appropriate then it will produce increased heat that will be transferred to cell electrodes and cause much faster battery cell degradation.
You can should always hard test all parts connected to your battery cells for higher temperatures under the max load (high amps) to know the exact issues that could be caused in your system.
Using appropriate Basbar and cable sizes you will prolong the life of your battery cells and reduce temperature. Higher temperatures can cause isolation issues and can even cause fires.
Using lower cable size is very dangerous and could cause isolation to melt and trigger fires.
Solar is not the same as EVs, as EVs in most cases use much more Amps/kW. If you can't match your busbar and cable size for required Amps then you should consider going to higher voltage.