Why should uppers be thinner than lowers?

[Brion Toss]

Hi again,
And oops, I mispoke. The lowers share about 55% of the total transverse load in this rig, so each of them takes less than half of that load. Because the load moves around some, due to things like reefing and dynamic motion, it is prudent to make them a bit stronger than 22.5% of the total, but each wire will still bear significantly less than the upper, so can be smaller.
In our example, if the uppers are 3/8" at 17500lbs, and that is something like 45% of the load (the upper's share), then 55% would be on the order of 21000lbs. Half of this is in the neighborhood of the strength of 5/16".
Is that clearer? Thank you for bearing with me.
Fair leads,
Brion Toss