Empirically, I think the current it takes to pull in a starter solenoid (all the way until the contacts are made) can be carried easily through a 14 gage wire with almost no voltage drop to the battery or along the positive side wiring battery through a switch to the solenoid energizing terminal.
If the connections are normal (this type of wiring survives decades in cars trouble free), the starter key switch or push button is rated for that same amount of current or greater and there are no other significant loads drawing through the wire upstream of the switch at the moment the switch is made, I just can't see what an even bigger switch capable of carrying multiples of the required current would do, unless there was a voltage sag produced by another action (locked rotor current momentarily going through the starter's winding). I am not an electron so I certainly can't say exactly whats going on, but providing enough current to pull in a starter solenoid coil all the way doesn't seem to me like it would present such a challenge that the switch would have to be upsized so drastically. Those relays are designed to carry enough current for an actual starter. Since it's a fact that there is enough battery amps available to actually turn the starter, it's just the fraction of those amps it takes to automatically make the circuit that seems tricky to provide reliably (to a little stakon terminal sticking out of the solenoid), and this seems like a problem that bedevils diesel boatowners year after year, I think there's more to it then the inadequate harness and switch. But that's just me.
If the connections are normal (this type of wiring survives decades in cars trouble free), the starter key switch or push button is rated for that same amount of current or greater and there are no other significant loads drawing through the wire upstream of the switch at the moment the switch is made, I just can't see what an even bigger switch capable of carrying multiples of the required current would do, unless there was a voltage sag produced by another action (locked rotor current momentarily going through the starter's winding). I am not an electron so I certainly can't say exactly whats going on, but providing enough current to pull in a starter solenoid coil all the way doesn't seem to me like it would present such a challenge that the switch would have to be upsized so drastically. Those relays are designed to carry enough current for an actual starter. Since it's a fact that there is enough battery amps available to actually turn the starter, it's just the fraction of those amps it takes to automatically make the circuit that seems tricky to provide reliably (to a little stakon terminal sticking out of the solenoid), and this seems like a problem that bedevils diesel boatowners year after year, I think there's more to it then the inadequate harness and switch. But that's just me.