Why should uppers be thinner than lowers?

[cmm]

Thank you for the input - I didn't know that 1x19 wire had to be pre-streched (like pre-stretched nylon rope) before or during initial use.

However, one can imagine that a Valiant 40 could handle the tightening of its lowers, whether they are 5/16" or 3/8" in diameter. So are there any other structural issues that may come into play?

Regards
Martin