Why should uppers be thinner than lowers?

[mholmes]

We just purchased a valiant 40 (Syzygy #201) and we are redoing the whole gang with wire using norseman fittings. The original manual that came with the boat gives the diameters of the lowers (two lowers each side) as 3/8", and uppers as 5/16".

It seems strange to me that the uppers should be weaker than the lowers. Is there a good explanation for this? Are not the uppers under much greater tension? Is there any reason I should change it?

much thanks for a stellar forum,
Matt