I think that the small conductor rule has been in the code for so long that nobody can remember the reason that it was put in the code in the first place.
Chris
The 'small conductor rule' first shows up in 1956, at the bottom of Table 1, Chapter 10 (it's now T310.16):
"The current-carrying capacities for Type RHH conductors for sizes 14, 12 and 10 shall be the same as designated for Type RH conductors in this Table." This put 14, 12 and 10 RHH at 15, 20 and 30 amps respectively. RH was a 75?C conductor, and RHH was a 90?C conductor.
Why? I don't know... he's on third.