You might want to look for another driller. This one is either trying to get to you or doesn’t have much experience. Variable Speed pumps are designed to take extra money out of your pocket. They cost a lot and don’t last very long. Yes you can use smaller wire with the three-phase motor, but the longer the wire the higher the voltage spikes from the VFD, and the sooner your motor will be fried. It’s called a reflective wave, and the longer your wire the worse the problem. I usually here from people about 3-4 years after they do this. They have gone through a couple new vfd controllers, a couple of new motors, and are tired of it costing so much and being out of water so frequently. So now they have to replace the smaller wire with larger wire, and replace the three-phase motor with a single-phase one, and basically start from scratch.
If your driller doesn’t know these things, then he/she just hasn’t had enough experience to know better. Or worse, they do know better and know this is a way to get you to pay for their next vacation or new drilling rig. The difference in the price of wire is well worth getting something dependable that will last a long time.
All the pump, motor, and controls manufacturers are really pushing variable speed pumps on their installers and on you. Do you really think they would spend so much advertising something that would save you money? NO! They advertise what makes them the most money, which means it cost you plenty.