Why should uppers be thinner than lowers?

[Brion Toss]

Weight, windage, and cost are far from trivial issues; any one of them might make for a compelling reason to go with smaller wire size. And the loads needed to tune oversize wire, as pointed out above, are not trivial either. But I don't get the feeling that we are talking about the same thing when we say "structural difference." If I understand you correctly, you think that objections to 3/8" lowers are minor compared to some imagined benefit(s). It is at times like these that I become aware of how evangelistic I can be on this subject. Really, I know I take it all too seriously. But what we are talking about here is a series of relationships in an astonishing, elegant machine. Nothing in a well-designed rig is arbitrary. It isn't a matter of having the wire break before something else, it is a matter of everything being in harmony. And there is no benefit in having it out of harmony. Yes, bigger wire is stronger, but what does that get you, if the loads don't require it, except more frequent reefing (more weight, more windage), more compression on the mast, more tension on the hull, an unsatisfactory tune, and thus worse performance, and more expense? Once you have met the loads, with whatever the appropriate safety factor is included, going bigger is not better.
I recommend going heavier where it is needed. If you want to hoist large heavy objects over people's heads, a safety factor of at least 10 to 1 makes sense, but safety is heavy, and a sailing vessel can only afford so much before the weight results in danger.
Think of this question as a window into your rig. Look at the whole thing, and discover the elegance that is available to you there.
Fair leads,
Brion Toss