This thread has been interesting and frustrating, perhaps another approach would help.
Long ago in the 70's I asked the much older at the bike shop, why are top tubes were 1" (allowing for the french minor difference)?
They did cite some of the non standard frames built by various builders, but could not provide hard answers, It was obvious the tube makers had settled on a 1'" top tube as typical, and the lug makers produced in quantity those dimensioned parts. But other than the at the time "rule" that 50:1 was a "safe" limit for steel tube diameter to wall thickness, no suitable answer was forthcoming.
Someone cited that frames long ago had a second horizontal tube below the top tube, during the early development of the "safety bicycle", perhaps that played a part in the derivation.
I did point out at the time 1973, that Teledyne Linar had as part of there advert. collateral for their new frame shown an image of a lugged bicycle being ridden on rollers and "wired" up for evaluating loads and flex and they had settled on a larger top and down tube diameter, leaving the seat tube "standard" to accept production components.
Some conjectured in regard to weight as being the reason, but not much on the why.
Of course today, a 1" top tube would probably be a special order from the tube supplier.
I found it interesting that few were questioning the wisdom of convention.
Anyone shed some light?
Torrance Ca USA