Obviously the situation is different with 3-phase because of the theoretical possibility of the neutral current being greater than the line. Usually it's less, often much less, but if much of the load has phase-angle control or generates heavy 3rd harmonic distortion, it can happen.
In single-phase circuits where the line and neutral currents must be equal, I agree that there is probably a human factor. People might habitually do the line first, then by the time they do the neutral they are thinking about the next bit and don't pay so much attention to making the termination. There might also be a difference in construction so that one terminal has more heatsinking than the other, but I would not say it's usually the line that has more metal attached.
There are certainly situations, especially DBs, where the terminals are totally different, but again I'm not conscious of neutral terminals being consistently poorer, except maybe in DBs where a simple screw terminal in the N compares to a cage-clamp in an MCB. OTOH the N has more cooling and less thermal cycling, so the benefit is possibly lost.
Another difference I thought about, was whether the insulation on the neutral has more of a tendency to leave tarnish or deposit on the surface of the copper that affects the connection. Clutching at straws though...
