Assuming that in all three cases the same amount of power would be lost in the wiring:

In the case of 3 lines (AC) coming down the tower vs. 2 (DC) coming the tower: yes, with 3 wires (more or less in parallel) you could use thinner wires and still end up with the same line loss. The new wires could be 20% or 25% less in diameter. I'm not sure which of the two figures is correct, still pondering that myself and won't get into detail here, as I'm already accused of thread hijacking. The difference is mainly academic anyway.

In the case of 6 lines (with star/delta at ground level)  vs. 3 lines coming down the tower, since the wires are now in series (2 sets of 3 wires), each of the 6 wires would actually have to be thicker than the wires in the 3-line-AC system. Since total length of the wires is now doubled, area of each wire should be double too (or in other words, diameter should be (sqrt(2)) = 1.41 as large.

At least, if I understand things correctly. If not I'll be no doubt corrected by others :)