Why should uppers be thinner than lowers?

[cmm]

Originally Posted by Brion Toss

Hi again
I prefer to put a heavy-duty thimble in, just as I would for wire. The radius of the clevis pin would be a bit minimal, and there would also be the possibility of chafe, unthimbled.
As for the shrouds, how about 5/16" for the lowers?
Fair leads,
Brion Toss

I apologize for entering this thread from the side, but my eye caught Brion's recommendation to keep the lowers 5/16 instead of 3/8.

In this game it is very easy for novices like me to think, "the thicker the better".

The only counter advice to oversizing my rigging wires I have heard, came from a fellow yacht owner who feels it is better that wire is the weakest link - if it was not, something more threatening to yacht integrity may suffer damage (e.g. chainplate may rip out or a bulkhead may be damaged) in the event of an abnormal stress.

Brion, is your advice to Matt to reduce his lowers form 3/8 to 5/16, at all related to the same consideration - i.e. if the wire is too strong something else may break, or is there a different reason?

Re-framing it as a more general question, heavily oversized rigging wire will resist breakage more; but are there any negative, structural consequences (setting aside things like wire and terminal costs, windage and weight)?

Thanks
Martin