One thing to consider is that any appliance wants to be able to draw it's design power from the supply lines. Not knowing how long those wires are, or how they are run, the codes specify a particular minimum wire gauge to ensure there's not an appreciable voltage drop at the maximum load possible based on both the wire size and the protection (typically either a fuse or a circuit breaker). If the wire size is too small, it will heat up, and the voltage will drop, neither one is a good proposition.
But, an appliance manufacturer knows how much power it will draw from that supply along with the expected voltage drops and temperature rise, and can then adjust the wires so that they will work in the appliance without issues. This often has the wires ending up smaller in an appliance than those feeding it. Think about a lamp cord...each one will have a maximum wattage that it can support, and that is based on the wire, switch, socket that are inside...those are almost never the same size as that feeding the receptacle it is plugged into, even if it is hard-wired. FWIW, you'll usually see the same thing on a water heater, or any other appliance. Bigger wires cost more, and may not buy you anything. But, you MUST ensure that your connections are made properly, as those can create heat and problems if they are not done right.
