How do you terminate 4/0 cable to a Xantrex C65 controller? Lugs aren't that big

From page 42 of the owners manual:

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Long-distance wire runs

If there is a significant distance between the PV array and the controller and/or the controller and the battery, larger wires can be used to reduce the voltage drop and improve performance. Refer to Table 2-4.

To use a larger size wire, use a splicer block (terminal block) intended for this purpose. This allows the larger cable size from the batteries to be “spliced” to the smaller wire size connected to the controller. Split-bolt kerneys can also be used for wire splices.

Follow manufactures recommendations for torque and mounting (if required). Splicer blocks and split-bolt kerneys are available from renewable energy suppliers. 

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The splice allows you the benefit of the reduction in Vdrop over the long run, then a transition to the smaller, ampacity rated wire at the charge controller.

Just a tip, when I was in Houston a few years ago and needed a lot of 4/0 & other large wire, but did not have a need for an exact kind of wire, I found a used wire warehouse that sold it to me at super low prices. They especially had large welding cables with very fine strand wire and thick, soft, rubber-like insulation. Perhaps neoprene.

There was a HUGE building and they knew where things were, so it was like going to a regular contractor supply, but at 10 cents on the dollar. Sometimes less.

So, wherever you are, look around and see what you can find. Just be sure that what they have is actually suitable for your needs, code, UV resistance, etc and most of all is it really safe?

And, as you hopefully know, connections and corrosion are your 2 worst enemies. Doing mostly marine work, I really like soldered connections, but if the poop hits the fan it can become unsoldered due to high current, so be sure to stake the connector properly before soldering and since you cannot use plastic coated crimp connectors that everyone uses on smaller wire. I use a good heatshrink when I am done. I crimp, solder, "butter it" with zinc oxide heatsink compound (zinc oxide in a high temp grease used for power transistors, etc) and then heatshrink. Many laugh at that, but I have recently replaced joints like that that I put on an ocean going boat in 1976. I cut them open to see how much corrosion there was as the yacht has spent most of its life in the tropics.

There was none! It looked like the day I did it. Seal the ends of the wires very well. SCOTCHKOTE 14853 and Scotch® 23 Tape - Premium Grade Self Bonding Rubber have worked really well for me for 40 yrs. I really like 3M in general, but I tried several "more up to date products" and could not stand them. They may be wonderful, but not for me. Once you find a good solution, stick with it. End of sermon