I'm an electrical engineering student (just need english and other liberal arts class to get my degree, done with all the engineering courses), and I've never heard of the term "voltage stealing". What I have heard of is the concept of a "voltage drop". The basic idea is that if you have a circuit with multiple things in it, each of which has some amount of electrical resistance (let's assume for simplicity that it's all in series), and let's say you power it with a 12 volt battery, and then you put a volt meter across any of these things, you'll measure a voltage that's smaller than 12 volts. If you measure the voltage across each of these things, they'll always add up to 12 volts.
So, let's say you have a circuit that goes from your 12 volt battery, through a loose, corroded wire that has a resistance of 2 ohms, and into your starter motor, which has a resistance of 1 ohm. Believe it or not, this is enough information to figure out everything we need to know about the circuit, thanks to a very simple, elementary equation known as Ohm's Law. It says that voltage is always equal to resistance times amperage. So, if we have 12 volts and 3 ohms total in the circuit, that means we will be drawing 4 amps from the battery, and those 4 amps are constant through the whole circuit since everything is connected in series, in one single line.
So, the voltage across the crappy corroded wire will be 8 volts and the voltage that actually gets to the starter motor will only be 4 volts. Because of that crappy wire, it's like you connected the starter motor straight to a 4 volt battery.
Now, these resistances aren't actually representative of what goes on in real life. It's actually much worse than the example I just gave. The starter motor will have a much lower resistance than 1 ohm. It'll try to pull several hundred amps and lower its resistance to maybe .05 ohms at the most.
So you'll want any wires going to the starter to be as low resistance as possible, since even if that wire is only a tenth of an ohm, which it probably is at least, two thirds of the power will be going into heating up that wire and only 1 third will be going to starting the car.
TLDR: Use as thick of a wire as you possibly can to replace that corroded one. You literally cannot use too thick of a wire. The larger the wire, the better your car will start. Make it as short as possible, as well.