I plan to build a little dinghy asap. It will be a sailing edition. But, from what I understand of the design, the stick is going to be solid. And there are two yards, as it is a balanced lug rig.
Basically, no matter the size of the craft the centre of weight should be as near to the waterline as possible (yes, this is a gross over-simplification, but it is a useful rule of thumb.) Everything tall or far away from the waterline needs to be as light as can be yet still perform its duties.
One traditional way of doing that is to use hollow spars. Most of the compression and tension forces are actually in the outer 20% of the a spar, so it makes sense to hollow them out because the inner portion is just dead weight.
Another is to taper the spars. A larger diameter is needed where the load is highest, the point of greatest bending forces, but a smaller diameter can be used where there is less bending force. The ends of a spar are generally mostly taking compressive force, so spars are often gently tapered toward their extremities. This trimmed-down shape simply has less material, and less drag, so it can help the boat’s sailing performance.
But, and this is important, in such a tiny boat it is unlikely the weight savings would be great enough to justify the increases in construction complexity, cost, and time. To borrow a coding phrase: ‘premature optimization’.
Still, it gives me yet another thing to noodle on while I wait until I can start building the thing. (Months!)