Atomic explanation as to why a piece of constantan wire with a large diameter has a smaller resistance than a piece of constantan wire with a small diameter?

Answer 1

The larger the wire the more atoms there are to transfer the electrons through the wire. Induced electrons from a power source bump other electrons out of the orbit of the wire atoms thus causing electrons to flow and the induced electrons fall into the orbit of the wire atom until the next loose electron knocks that one out of orbit to the next atom and the next one etc.. It's like a one way game of bumper cars until the electrons get to the end of the wire and into the next conducting substance. This is also the difference between an insulator and a conductor that is how freely an atom gives up its electron when its bumped.

Answer 2

A larger diameter constantan wire has more space for electrons to move through, reducing the chances of collisions with the atoms in the wire. This results in lower resistance because there are fewer obstacles for the electrons to overcome as they flow through the wire. Conversely, a smaller diameter wire has less space, leading to more collisions and higher resistance.