A teacher explained why by using a highway analogy. The more lanes you have, the faster the cars go through, where the number of lanes obviously represent the wire thickness and the cars represent electrons. Easy enough.
But after a certain point shouldn't the wire get so thick, that any thickness after that doesn't affect resistance? For example, if you have a 100 cars going down a highway, a 4 lane highway is going to allow the cars to move a lot faster than a 1 lane one, because there are a fewer cars per lane. But a 1000 lane highway highway is going to be as efficient as a 10000 lane one, because on both highways every car has its own lane. After a 100 lanes, the number of lanes doesn't provide resistance.
So why does increasing wire thickness always decrease resistance?
Best Answer
The car analogy isn't such a good one, since electrons don't actually flow from one end of the wire to the other (well they do but extremely slowly) and it implies there is some space between the cars, whereas it would be more like a traffic jam whatever the width of the highway.
It's more like a line of billiard balls, and force is applied to the first one, and the energy is transferred to the last one through all the intermediate balls (a bit like newtons cradle, although the balls don't really bounce into each other). The free electrons bounce around, occasionally being impeded (see below) with the potential difference causing an average inclination to the direction of current.
A water analogy is better - the pipe is always full of water, and for the same pump (battery), the pressure (voltage) is always lower the wider the pipe, which equates to more flow and a lower resistance.
This quote from the Wiki page on resistivity explains reasonably well: