My hunch is that the Hong Kong style junk sail, in particular the mainsail, is a result of this shortage of suitable timber for masts, which Gary points out. I have not seen any JR from China with a bigger sail area set on shorter masts than those mainsails in Hong Kong. These short masts also give a wider angle between shrouds and the masts, so it makes more sense to add shrouds to them.
Besides, the Hong Kong style mainsail is as un-balanced as the gaff cat-rig sails in the US. When sailing close-hauled, the sheet will do its best to bend the mast aft, so it is no surprise that the shrouds seem to be set a bit forward (many catboats had a forestay for the same reason).
Also note the hard-working Hong Kong parrels of these sails - they clearly bend the stout battens upwards. I guess these were compromises the sailors were willing to live with to save cost (and weight?) of the main mast. The rigs surely appear to work well.
One of the fine things with wooden masts is that they need not be circular in section. I have had more than one of them made slightly oval with them being 10% thicker in the alongship direction. Now that I make hybrid aluminium-wood masts, I aim for a yield strength twice the max righting moment of the boat. For serious offshore boats I would aim for a factor of 3. As the boats grow in size, these high strength factors will be more difficult to achieve, so I guess one must be more stress conscious when sailing bigger vessels.
Arne