For example Makani M 600 span is 28 m, and the swept area diameter is 270 m, so between 9 and 10 times the wing span. One reason is the optimization of the wing. If the swept area diameter is far lower the internal tip wing speed will be far lower, involving in a lower efficiency. As comparison a ground-based wind rotor diameter is only two times a blade span but the blades design is enough optimized so that Betz limit can be approached. Deducing 9 times wing span swept area diameter doesn’t allow optimizing Betz limit (I put also the link with a discussion in regard to Betz limit) with only one wing, even with two wings. It is the reason why (in spite of its 4/27 limit) a yoyo system remains competitive until more drag wings can safely be installed.